Stabilized phosphate ester-based functional fluid compositions

ABSTRACT

A fluid composition suitable for use as an aircraft hydraulic fluid is disclosed. The fluid composition comprises a fire resistant phosphate ester base stock comprising between about 10% and about 100% by weight of a trialkyl phosphate, between about 0% and about 70% by weight of a dialkyl aryl phosphate, and from about 0% to about 25% by weight of an alkyl diaryl phosphate, with the proviso that the sum of the proportionate amount of each base stock component must equal 100%. The alkyl substituents of the trialkyl phosphate, the dialkyl aryl phosphate, and the alkyl diaryl phosphate contain between 3 and 8 carbon atoms, preferably between 4 and 8 carbon atoms, more preferably between 4 and 5 carbon atoms, and are bonded to the phosphate moiety via a primary carbon. It is still further preferred that the alkyl substituents of the trialkyl phosphate, the dialkyl aryl phosphate, and the alkyl diaryl phosphate are isoalkyl groups. The fluid composition further comprises an acid scavenger, an anti-erosion additive, a viscosity index improver, and an antioxidant. A novel additive combination comprises a high molecular weight butyl/hexyl methacrylate viscos ity index improver, a perfluoroalkylsulfonate anti-erosion additive, a 3,4-epoxycyclohexanecarboxylate or a diepoxide acid scavenger, a di(alkylphenyl)amine, and a phenolic antioxidant comprising a mixture of a 2,4,6-trialkylphenol and a hindered polyphenol compound selected from the group consisting of bis(3,5-dialkyl-4-hydroxyaryl)methane, 1,3,5-trialkyl-2,4,6-tris(3,5-di-tert-butyl-4-hydroxyaryl)benzene and mixtures thereof. Preferably, the fluid composition further comprises a benzotriazole derivative as a copper corrosion inhibitor, and a 4,5-dihydroimidazole derivative, as an iron corrosion inhibitor and to enhance the stability of the fluid. The issued reissue patent is the reexamination certificate.

This is a continuation in part of Ser. No. 07/897,189 filed Jun. 11,1992, now abandoned.

BACKGROUND OF THE INVENTION

This invention relates to phosphate ester functional fluids and moreparticularly to phosphate ester fluids of improved thermal, hydrolyticand oxidative stability useful as aircraft hydraulic fluids.

Functional fluids have been utilized as electronic coolants, diffusionpump fluids, lubricants, damping fluids, bases for greases, powertransmission and hydraulic fluids, heat transfer fluids, heat pumpfluids, refrigeration equipment fluids, and as a filter media forair-conditioning systems. Hydraulic fluids intended for use in thehydraulic system of aircraft for operating various mechanisms andaircraft control systems must meet stringent functional and userequirements. Among the most important requirements of an aircrafthydraulic fluid is that it be stable against oxidative and hydrolyticdegradation at elevated temperatures.

In use, aircraft hydraulic fluids commonly become contaminated withmoisture. Water enters the hydraulic system with air bled from an enginecompressor stage. During operations, the moisture level in Type IVaircraft hydraulic fluids normally ranges from about 0.2 to about 0.35%by weight. Water causes hydrolytic decomposition of phosphate esters toproduce partial esters of phosphoric acid. Hydrolytic breakdown of theester is accelerated if water content exceeds about 0.5% by weight.Conventionally, phosphate ester aircraft hydraulic fluids are formulatedto contain an acid scavenger which neutralizes partial esters ofphosphoric acid released by hydrolytic breakdown of the triester. Overtime, however, the acid scavenger becomes depleted and organometalliccompounds are formed by complex reactions involving the phosphatetriester, phosphoric acid partial esters, and surfaces of the metalenvironment within which the hydraulic fluid is ordinarily contained.These organometallic compounds, of which iron phosphate is usually themost prominent by-product, are not soluble in the hydraulic fluid.

Higher performance aircraft are operated under conditions which exposehydraulic fluids to increasing temperatures. Current Grade A fluidsoperate at maximum temperatures in the range of 225 to 240° F. However,projected aircraft applications will expose aircraft hydraulic fluids tobulk fluid temperatures in the range of 275° F. or higher. At suchtemperatures, the potential for oxidative and hydrolytic breakdown ofphosphate esters is substantially increased.

Degradation of phosphate ester hydraulic fluids is also acceleratedwhere the fluids are exposed to compressed air. The rate of airoxidation of such fluids also increases with temperature. Thus, forapplication at 275° F. or higher, a need exists for fluids of bothenhanced thermal oxidative stability and enhanced thermal hydrolyticstability.

Erosion problems may also be expected to increase with bulk fluidtemperature. Erosion is a form of electrochemical corrosion, moreprecisely referred to as zeta corrosion, the rates of which areincreased with temperature. The incidence of cavitation, which is one ofthe mechanical sources of erosion problems, is also likely to increasewith temperature. As erosion progresses, the presence of metallic orother insoluble components may result in filter clogging andreplacement, and can cause a change in the physical and chemicalproperties of the fluid, thereby requiring premature draining of fluidsfrom the system. Metal contaminants also reduce oxidative stability ofthe fluid, accelerating corrosion. In addition to any effects resultingfrom contamination by metal (or other) contaminants, the fluid maysuffer deterioration in numerous other ways, including: a) viscositychange; b) increase in acid number; c) increased chemical reactivity;and d) discoloration.

A hydraulic fluid useful in aircraft is available from applicants'assignee under the trademark Skydrol® LD4. This composition contains 30to 35% by weight dibutyl phenyl phosphate, 50 to 60% by weight tributylphosphate, 5 to 10% of viscosity index improvers, 0.13 to 1% of adiphenyldithioethane copper corrosion inhibitor, 0.005% to about 1% byweight, but preferably 0.0075% to 0.075% of a perfluoroalkylsulfonicacid salt antierosion agent, 4 to 8% by weight of an acid scavenger ofthe type described in U.S. Pat. No. 3,723,320 and about 1% by weight of2,6-di-tertiary-butyl-p-cresol as an antioxidant. This composition hasproved highly satisfactory in high performance aircraft application.However, it was not designed for extended operations at temperatures inthe range of 275° F.

SUMMARY OF THE INVENTION

Among the several objects of the present invention, therefore, may benoted the provision of an improved functional fluid useful as ahydraulic fluid in aircraft applications; the provision of such a fluidwhich exhibits improved hydrolytic stability, especially at elevatedtemperatures; the provision of such a fluid which exhibits improvedoxidative stability at elevated temperatures; the provision of such afluid which exhibits advantageous viscosity characteristics andespecially viscosity stability under shear conditions; the provision ofsuch a fluid of relatively low density; the provision of such a fluidwhich has not only high resistance to oxidation but also low toxicity;the provision of such a composition which has improved anti-erosionproperties; and the provision of such a fluid composition which exhibitsimproved resistance to corrosion of metal components of an aircraft orother hydraulic fluid system.

Briefly, therefore, the present invention is directed to a fluidcomposition suitable for use as an aircraft hydraulic fluid. Thecomposition comprises a fire resistant phosphate ester base stock, thebase stock comprising between about 10% and about 100%, preferablybetween about 20% and about 99%, by weight of a trialkyl phosphate,between about 0% and about 70% by weight of a dialkyl aryl phosphate,and from about 0% to about 25% by weight of an alkyl diaryl phosphate,with the proviso that the sum of the proportionate amount of each basestock component must equal 100%. The alkyl substituents of the trialkylphosphate, the dialkyl aryl phosphate, and the alkyl diaryl phosphatecontain between 3 and 8 carbon atoms, preferably between 4 and 8 carbonatoms, more preferably between 4 and 5 carbon atoms, and are bonded tothe phosphate moiety via a primary carbon. It is still further preferredthat the alkyl substituents of the trialkyl phosphate, the dialkyl arylphosphate, and the alkyl diaryl phosphate are isoalkyl groups. In apreferred embodiment, the base stock of the composition comprisesbetween about 50% and about 85% by weight of a trialkyl phosphate,between about 18% and about 35% by weight of a dialkyl aryl phosphate,and from 0 to about 5% by weight of an alkyl diaryl phosphate. Inaddition to the fire resistant base stock, the composition furthercomprises an acid scavenger in an amount effective to neutralizephosphoric acid partial esters formed in situ by hydrolysis of any ofthe phosphate esters of the base stock; an anti-erosion additive in anamount effective to inhibit flow-induced electrochemical or zetacorrosion of the flow metering edges of hydraulic servo valves inhydraulic systems; a viscosity index improver in an amount effective tocause the fluid composition to exhibit a viscosity of at least about 3.0centistokes (cst) at about 210° F., at least about 9.0 centistokes atabout 100° F., and less than about 4200 centistokes at −65° F.; and ananti-oxidant in an amount effective to inhibit oxidation of fluidcomposition components in the presence of oxygen.

Preferably, as previously indicated, the alkyl substituents of thetrialkyl phosphate, dialkyl aryl phosphate, and the alkyl diarylphosphate contain between 4 and 8 carbon atoms, more preferably between4 and 5 carbon atoms. It is still further preferred that the alkylsubstituents of the trialkyl phosphate, the dialkyl aryl phosphate, andthe alkyl diaryl phosphate are isoalkyl groups. Most preferably,therefore, the alkyl substituents are isoalkyl C₄ and C₅ groups, namely,isobutyl and isopentyl (also known as isoamyl), respectively.

The invention is further directed to a fluid composition suitable foruse as an aircraft hydraulic fluid and containing a novel combination ofadditives. The composition comprises a fire resistant phosphate esterbase stock comprising between about 10% and about 90% by weight of atrialkyl phosphate, between about 0 and about 70% by weight of a dialkylaryl phosphate and from 0% to about 25% by weight of an alkyl diarylphosphate. The alkyl substituents of the trialkyl phosphate, the dialkylaryl phosphate, and the alkyl diaryl phosphate contain between 3 and 8carbon atoms, preferably between 4 and 8 carbon atoms, more preferablybetween 4 and 5 carbon atoms and are bonded to the phosphate moiety viaa primary carbon atom. It is still further preferred that the alkylsubstituents of the trialkyl phosphate, the dialkyl aryl phosphate, andthe alkyl diaryl phosphate are isoalkyl groups. The composition furthercomprises a viscosity index improver in a proportion, on a solids(methacrylate polymer, as hereinafter described) basis, of between about3% and about 10% by weight of the composition. The viscosity indeximprover comprises a methacrylate ester polymer, the repeating units ofwhich substantially comprise butyl and hexyl methacrylate, at least 95%by weight of the polymer having a molecular weight of between about50,000 and about 1,500,000. The viscosity index improver is convenientlyemployed or provided in the form of a solution in a phosphate estersolvent, preferably a trialkyl phosphate ester, such as, for example,tributyl or triisobutyl phosphate, or a combination of alkyl and arylphosphate esters. In a preferred embodiment, the phosphate ester solventis comprised of one or more of the phosphate ester components whichconstitute the phosphate ester base stock of choice. In such manner, thephosphate ester solvent becomes in effect part of the base stock, andthe stated ranges of suitable proportions of phosphate estershereinafter described reflect the phosphate ester(s) added as a carrieror vehicle for the viscosity index improver. The composition furthercomprises an anti-erosion agent in a proportion of between about 0.02%and about 0.08% by weight of the composition, the anti-erosion agentcomprising an alkali metal salt of a perfluoroalkylsulfonic acid, thealkyl substituent of which is hexyl, heptyl, octyl, nonyl or decyl. Thecomposition comprises an acid scavenger in a proportion of between about1.5 and about 10% by weight of the composition, the acid scavengercomprising a derivative of 3,4-epoxycyclohexane carboxylate or adiepoxide compound of the type disclosed in U.S. Pat. No. 4,206,067. Thecomposition further contains a 2,4,6-trialkylphenol in a proportion ofbetween about 0.1% and about 1% by weight, a dialkylphenyl)amine in aproportion of between about 0.3% and about 1% by weight, and a hinderedpolyphenol compound selected from the group consisting ofbis(3,5-dialkyl-4-hydroxyaryl)methane, 1,3,5-trimethyl-2,4,6-tris(3,5-di-tert-butyl-hydroxyaryl)benzene and mixtures thereof in aproportion of between about 0.3% and about 1% by weight of thecomposition. The alkyl substituents of trialkyl phosphate and dialkylaryl phosphate are preferably butyl or pentyl.

The invention is further directed to a fluid composition suitable foruse as an aircraft hydraulic fluid comprising a fire resistantorganophosphate ester base stock. The base stock comprises between about10% and about 100%, preferably between about 20% and about 99%, byweight of a trialkyl phosphate wherein the alkyl substituents aresubstantially isoalkyl C₄ or C₅, between about 0% and about 70% byweight of a dialkyl aryl phosphate wherein the alkyl substituents aresubstantially isoalkyl C₄ or C₅, and between about 0% and about 25% byweight of an alkyl diaryl phosphate wherein the alkyl substituent issubstantially isoalkyl C₄ or C₅. The composition further comprises anacid scavenger in an amount effective to neutralize phosphoric acid andphosphoric acid partial esters formed in situ by hydrolysis of any ofthe phosphate esters of the base stock; an anti-erosion additive in anamount effective to inhibit flow-induced electrochemical corrosion ofthe flow metering edges of hydraulic servo valves in hydraulic systems;a viscosity index improver in an amount effective to cause the fluidcomposition to exhibit a viscosity index of at least about 3.0centistokes at about 210° F., at least about 9.0 centistokes at about100° F., and less than about 4200 centistokes at about −65° F.; and anantioxidant in an amount effective to inhibit oxidation of fluidcomposition components in the presence of oxygen.

The invention is further directed to a fluid composition suitable foruse as an aircraft hydraulic fluid comprising a phosphate ester basestock. The base stock comprises between about 10% and about 100%,preferably between about 20% and about 99%, by weight of a trialkylphosphate wherein the alkyl substituents are substantially C₄ or C₅,preferably isoalkyl C₄ or C₅ (namely, isobutyl or isopentyl), betweenabout 0% and about 70% by weight of a dialkyl aryl phosphate wherein thealkyl substituents are substantially C₄ or C₅, preferably isoalkyl C₄ orC₅ (namely isobutyl or isopentyl), and between about 0% and about 25% byweight of an alkyl diaryl phosphate wherein the alkyl substituent issubstantially C₄ or C₅, preferably isoalkyl C₄ or C₅ (namely isobutyl orisopentyl). The composition further comprises an acid scavenger in anamount effective to neutralize phosphoric acid partial esters formed insitu by hydrolysis of any of the phosphate esters of the base stock; ananti-erosion additive in an amount effective to inhibit flow-inducedelectrochemical or zeta corrosion of the flow metering edges ofhydraulic servo valves in hydraulic systems; a viscosity index improverin an amount effective to cause the fluid composition to exhibit aviscosity index of at least about 3.0 centistokes at about 210° F., atleast about 9.0 centistokes at about 100° F., and less than about 4200centistokes at −65° F.; an antioxidant in an amount effective to inhibitoxidation of fluid composition components in the presence of oxygen; anda 4,5-dihydroimidazole compound in an amount effective to decrease by atleast about 25% the rate of breakdown at 300° F. of phosphate triestersin the composition to phosphoric acid partial esters, as measured byepoxide depletion. The 4,5-dihydroimidazole compound corresponds to theformula

where R¹ is hydrogen, alkyl, alkenyl, hydroxyalkyl, hydroxyalkenyl,alkoxyalkyl or alkoxyalkenyl, and R² is alkyl, alkenyl or an aliphaticcarboxylate. The invention is further directed to a fluid compositionsuitable for use as an aircraft hydraulic fluid comprising a fireresistant phosphate ester base stock. The base stock comprises betweenabout 10% and about 100%, preferably between about 35% and about 99%, byweight of a trialkyl phosphate, between about 0% and about 35% by weightof a dialkyl aryl phosphate, and between about 0% and about 20% byweight of a triaryl phosphate. The alkyl substituents of the trialkylphosphate and the dialkyl aryl phosphate contain between 3 and 8 carbonatoms, preferably between 4 and 8 carbon atoms, more preferably between4 and 5 carbon atoms and are bonded to the phosphate moiety via aprimary carbon. It is still further preferred that the alkylsubstituents of the trialkyl phosphate and the dialkyl aryl phosphateare isoalkyl groups. The aryl substituents of the dialkyl aryl phosphateesters and the triaryl phosphate esters are typically phenyl, but mayalso be an alkyl-substituted phenyl (alkylphenyl) wherein the alkylsubstituent is C₁ to C₉, preferably C₃ to C₄. Nonlimiting examples ofthe alkyl-substituted phenyl substituents include tolyl (also known asmethylphenyl), ethylphenyl, isopropylphenyl, isobutylphenyl,tert-butylphenyl, and the like. The fluid composition further comprisesan acid scavenger in an amount effective to neutralize phosphoric acidand phosphoric acid partial esters formed in situ by hydrolysis of anyof the phosphate esters of the base stock, an anti-erosion additive inan amount effective to inhibit flow-induced electrochemical or zetacorrosion of the flow metering edges of hydraulic servo valves inhydraulic systems; a viscosity index improver in an amount effective tocause the fluid composition to exhibit a viscosity of at least about 3.0centistokes at about 210° F., at least about 9.0 centistokes at about100° F., and less than about 4200 centistokes at −65° F.; and anantioxidant in an amount effective to inhibit oxidation of fluidcomposition components in the presence of oxygen.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1 to 12 are plots of epoxide depletion versus time for hydraulicfluid formulations tested under varying conditions of temperatures,moisture content, and other parameters; and

FIG. 13 is a bar graph illustrating the superior anti-corrosionproperties of the functional fluid of the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

In accordance with the present invention, it has been discovered that ahydraulic fluid of improved thermal, hydrolytic, and oxidative stabilityis provided by utilizing a phosphate ester base stock which contains ahigh concentration of alkyl ester moieties and contains relatively smallproportions of phenyl or other aryl esters.

The base stock comprises a mixture of trialkyl phosphate and dialkylaryl phosphate, in each of which the alkyl substituents are C₅ to C₈,preferably C₄ to C₈, more preferably C₄ or C₅, and are bonded to thephosphate moiety via a primary carbon. It is still further preferredthat the alkyl substituents of the trialkyl phosphate and the dialkylaryl phosphate are isoalkyl groups. Optionally, the base stock furthercomprises a small proportion of alkyl diaryl phosphate wherein the alkylsubstituent is as previously defined. Further advantages are realized ifthe alkyl substituents of the trialkyl phosphate, the dialkyl arylphosphate, and the alkyl diaryl phosphate esters are primarily comprisedof isoalkyl C₄ or C₅ (namely, isobutyl or isopentyl), in preference tothe normal isomers thereof. In this preferred instance also, attachmentof the alkyl substituent to the phosphate should be via a primary carbonatom.

In addition to the improved base stock, the composition of the inventionpreferably contains a combination of additives which further enhancesthe properties of the fluid as compared to fluids previously availablein the art for use in the aircraft hydraulic systems. Moreover, it hasbeen found that the additive combinations of this invention areeffective in enhancing the properties of base stock compositionspreviously known in the art or otherwise differing from the preferredbase stock of the functional fluids of this invention. But the mostadvantageous properties are realized using both the additive package andthe base stock of the invention. This is particularly true where thealkyl substituents of the trialkyl phosphate, the dialkyl arylphosphate, and the alkyl diaryl phosphate, especially the trialkylphosphate and the dialkyl aryl phosphate, are isoalkyl C₄ or C₅ (namely,isobutyl or isopentyl).

In a preferred embodiment, the base stock is characterized by a very lowalkyl diaryl phosphate ester content, preferably not more than about 5%by weight, more preferably not more than about 2% by weight. It isfurther preferred that the sum of the proportions of esters containingan aryl substituent, i.e., dialkyl aryl, alkyl diaryl, and triarylphosphates, does not constitute more than about 25% by weight of thebase stock.

More particularly, in a preferred embodiment, the base stock compositionadvantageously comprises between about 50% and about 85% by weight of atrialkyl phosphate wherein the alkyl substituents are substantially C₄or C₅, preferably isoalkyl C₄ or C₅ (namely, isobutyl or isopentyl),between about 18% and about 35% by weight of a dialkyl aryl phosphatewherein the alkyl substituent is substantially C₄ or C₅, preferablyisoalkyl C₄ or C₅ (namely, isobutyl or isopentyl), and from 0 to about5% by weight of an alkyl diaryl phosphate wherein the alkyl substituentis substantially C₄ or C₅, preferably isoalkyl C₄ or C₅, (namely,isobutyl or isopentyl). Preferably the aryl substituents are phenyl andalkyl-substituted phenyl (alkylphenyl) wherein the alkyl substituent isC₁ to C₉, more preferably C₃ to C₄. Non-limiting examples of thealkyl-substituted phenyl include tolyl, ethylphenyl, isopropylphenyl,isobutylphenyl, tert-butylphenyl, and the like, with tert-butylphenylgenerally being more preferred. As contrasted, for example, withSkydrol® LD-4 hydraulic fluid, which has a significantly higher diaryl(as diphenyl) ester content, the base stock of the functional fluid ofthe present invention exhibits significantly improved hydrolyticstability at temperatures substantially above 225° F. using the sameacid scavenger system as that incorporated into LD-4. Using the sameanti-oxidant additive as LD-4, a composition comprising the base stockof the present invention exhibits significantly enhanced thermal,oxidative, and hydrolytic stability. As a result of the relatively lowdiaryl ester content of the base stock, the functional fluid of thepresent invention has relatively low density, which is advantageous inaircraft hydraulic fluid applications.

In the preferred base stock of the present invention, it is particularlypreferred that the alkyl substituents be isoalkyl C₄ or C₅ (namely,isobutyl or isopentyl), most preferably isoalkyl C₄ (isobutyl). It hasbeen found that a base stock composition comprising triisobutylphosphate or triisopentyl phosphate and diisobutyl phenyl phosphate ordiisopentyl phenyl phosphate affords multiple advantages as compared tothe same compositions in which the alkyl substituents are n-butyl orn-pentyl.

TABLE A TBP TIBP Oral LD₅₀ 1200 mg/kg >5000 mg/kg Dermal LD₅₀ >10,000mg/kg >5000 mg/kg Eye Irritation Mildly irritating Practically non-irritating Skin Irritation Severely irritating Moderately irritat- ingSubchronic Bladder Hyperplasia In rats >1000 ppm None observed Inrats >5000 ppm NOEL 200 ppm NOEL 5000 ppm Hen Neurotox Not neurotoxicNot neurotoxic Tested at LD₅₀ = Tested at LD₅₀ = 1500 mg/kg >5000 mg/kgGenotoxicity Ames - negative Ames - negative CHO/HGPRT - Mousemicronucleus - negative negative in vitro cyto- genetics - negative invivo cyto- genetics - negative

In addition, in the context of the present invention, the phosphateesters wherein the alkyl substituents attached to the phosphate moietyare isoalkyl C₄ or C₅ (namely, isobutyl or isopentyl) have further beenfound to exhibit hydrolytic stability superior to that exhibited by thecorresponding normal alkyl phosphate esters at the high temperatures towhich the hydraulic systems of high performance aircraft are exposed.The realization of this advantage occurs in the absence of any adverseeffects upon seal integrity. That is, isobutyl and isopentyl estersmaintain the same high level of seal integrity exhibited by normal alkylphosphate esters. Or stated differently, the materials of whichhydraulic system seals are commonly fabricated have been found tomaintain a level of swelling when in contact with the isoalkyl estersthat is equivalent to that experienced when in contact with thecorresponding normal alkyl esters. Moreover, it has been found that theisobutyl and isopentyl esters are even lower density than the normalalkyl esters, which means that the weight of fluid in a given aircrafthydraulic system is lower, resulting in improved aircraft fuelefficiency.

In addition to the improved base stock, the composition of the inventionpreferably contains a combination of additives which further enhancesthe properties of the fluid as compared with fluids previously availablein the art for use in aircraft hydraulic systems.

More particularly, the composition incorporates an acid scavenger in aproportion sufficient to neutralize phosphoric acid and phosphoric acidpartial esters formed in situ by hydrolysis of components of thephosphate ester base stock under conditions of the service in which thehydraulic fluid composition is used. Preferably, the acid scavenger is a3,4-epoxycyclohexane carboxylate composition of the type described inU.S. Pat. No. 3,723,320. Also useful are diepoxides such as thosedisclosed in U.S. Pat. No. 4,206,067 which contain two linkedcyclohexane groups to each of which is fused an epoxide group. Suchdiepoxide compounds correspond to the formula:

wherein R³ is an organic group containing 1 to 10 carbon atoms, from 0to 6 oxygen atoms and from 0 to 6 nitrogen atoms, and R⁴ through R⁹ areindependently selected from among hydrogen and aliphatic groupscontaining 1 to 5 carbon atoms. Exemplary diepoxides include3,4-epoxycyclohexylmethyl-3,4-epoxycyclohexane, bis(3,4-epoxy-6-methylcyclohexylmethyl adipate),2-(3,4epoxycyclohexyl)-5,5-spiro(3,4-epoxy)cyclohexane-m-dioxane. Theconcentration of the acid scavenger in the fluid composition ispreferably between about 1.5% and about 10%, more preferably betweenabout 2% and about 8% by weight, which is generally sufficient tomaintain the hydraulic fluid in a serviceable condition for up toapproximately 3000 hours of aircraft operation.

To limit the effect of temperature on viscosity, the composition furtherincludes a polymeric viscosity index improver. Preferably, the viscosityindex improver comprises a poly(alkyl methacrylate) ester of the typedescribed in U.S. Pat. No. 3,718,596. Generally, the viscosity indeximprover is of high molecular weight, having a number average molecularweight of between about 50,000 and about 100,000 and a weight averagemolecular weight of between about 200,000 and about 300,000. Preferably,the viscosity index improver of the invention has a relatively narrowrange of molecular weight, approximately 95% by weight of the viscosityindex improver component having a molecular weight of between about50,000 and about 1,500,000. This result is achieved in part byutilization of predominantly butyl and hexyl methacrylate esters. Theviscosity index improver is present in a proportion sufficient to imparta kinematic viscosity of: at least about 3.0, preferably between about 3and about 5 centistokes at 210° F.; at least about 9, preferably betweenabout 9 and about 15 centistokes at 100° F.; and not more than about4200 centistokes at −65° F. Superior shear stability characteristics arealso imparted by the viscosity index improver used in the composition.Preferably the fluid composition contains between about 3% and about 10%by weight of the viscosity index improver. Nonlimiting particularlypreferred viscosity index improvers are those sold under the tradedesignations PA6703, PA6477, and PA6961-PMN by Rohm and Haas Company.The viscosity index improver, as previously noted in the Summary of theInvention, is conveniently provided in the form of a solution in aphosphate ester solvent, preferably a trialkyl phosphate ester such astributyl or triisobutyl phosphate, or a combination of alkyl and phenylderivatives. The proportions referred to above for the viscosity indeximprover are on a solids (methacrylatc polymer) basis. The phosphateester solvent becomes in effect part of the base stock, and the rangesof proportions of phosphate esters, as discussed above, reflect thephosphate ester added as a vehicle for the viscosity index improver.

An anti-erosion agent is incorporated in an amount effective to inhibitflow-induced electrochemical corrosion, more precisely referred to aszeta corrosion. The anti-erosion additive is preferably an alkali metalsalt, more preferably a potassium salt of a perfluoroalkylsulfonic acid.Such anti-erosion additives are more fully described in U.S. Pat. No.3,679,587. Typically, the alkyl component comprises hexyl, heptyl,octyl, nonyl, decyl, or mixtures thereof, with perfluorooctyl generallyaffording the best properties. It is particularly preferred that theanti-erosion agent predominantly comprises the potassium salt ofperfluorooctylsulfonic acid in a proportion of between about 250 andabout 1000 most preferably at least about 500 ppm. In the operation ofan aircraft hydraulic fluid system, the sulfonic acid moiety of theanti-erosion agent tends to lower the surface tension of the hydraulicfluid and thereby better cover the metal surfaces with which thehydraulic fluid normally comes in contact. The metering edges of servovalves are generally the most important metal parts which needprotection from electrochemical corrosion. Positive ions in the fluid,including the alkali metal ion of the anti-erosion agent, are adsorbedonto the metal surface and neutralize the negative charges on the metalthat are otherwise created by the rapid flow of the hydraulic fluid overthe servo valve metering edges. Enhanced erosion resistance is providedin the composition of the invention, which preferably contains aperfluoroalkylsulfonic salt content about twice that of the prior artcomposition sold as LD4.

Limiting the diaryl ester content of the base stock contributes tothermal, oxidative, and hydrolytic stability of the fluid. Thecomposition of the invention also contains a combination of antioxidantadditives, preferably including both a hindered phenol and a hinderedpolyphenol. Hydrolytic stability has been found to be improved bypartially substituting the hindered polyphenol for the phenol, and it isthus preferred that the composition contain not more than about 1.0%,preferably not more than about 0.7% by weight of a phenol such as a2,4,6-trialkylphenol. It is generally preferred that the compositioncontain between about 0.1% and about 0.7% of a 2,4,6-trialkylphenol,preferably 2,6-di-tertiary-butyl-p-cresol [also written as2,6di-tert-butyl-p-cresol or 2,6-di-t-butyl-p-cresol (“Ionol”)]. Thecomposition should further include between about 0.3% and about 1% of ahindered polyphenol compound, such as a bis(3,5-dialkyl-4-hydroxyaryl)methane, for example, the bis(3,5-di-tert-butyl-4-hydroxyphenyl)methanesold under the trade designation Ethanox® 702 by the Ethyl Corp., a1,3,5-trialkyl-2,4,6-tris(3,5 -dialkyl-4-hydroxyaryl) aromatic compound,for example, the 1,3,5-trimethyl-2,4,6-tris(3,5-di-tert-butyl-4-hydroxyphenyl)benzene sold under the tradedesignation Ethanox® 330 by the Ethyl Corp., or mixtures thereof. Thecomposition may also include an amine antioxidant, preferably adiarylamine such as, for example, phenyl-α-napthylamine oralkylphenyl-α-naphthylamine, or the reaction product ofN-phenylbenzylamine with 2,4,4-trimethylpentene sold under the tradedesignation Irganox® L-57 by Ciba-Geigy; diphenylamine, ditolylamine,phenyl tolylamine, 4,4′-diaminodiphenylamine, di-p-methoxydiphenylamine,or 4-cyclohexylaminodiphenylamine; a carbazole compound such asN-methylcarbazole, N-ethylcarbazole, or 3-hydroxycarbazole; anaminophenol such a N-butylaminophenol, N-methyl-N-amylaminophenol, orN-isooctyl-p-amino-phenol; an aminodiphenylalkane such asamino-diphenylmethanes, 4,4′-diaminodiphenylmethane, etc.,aminodiphenylethers; aminodiphenyl thioethers; aryl substitutedalkylenediamines such as 1,2-di-o-toluidoethane, 1,2-dianilinoethane, or1,2-dianilinopropane; aminobiphenyls, such as 5-hydroxy-2-aminobiphenyl,etc.; the reaction product of an aldehyde or ketone with an amine suchas the reaction product of acetone and diphenylamine; the reactionproduct of a complex diarylamine and a ketone or aldehyde; a morpholinesuch as N-(p-hydroxyphenyl)morpholine, etc., an amidine such asN,N′-bis-(hydroxyphenyl)acetamidine or the like; an acridan such as9,9′-dimethylacridan, a phenathiazine such as phenathiazine,3,7-dibutylphenathiazine or 6,6-dioctylphenathiazine, a cyclohexylamine,or mixtures thereof. An alkyl substituted diphenylamine such asdi(p-octylphenyl) amine is preferred. Certain amine components can alsoact as a lubricating additive. The amine antioxidant is also preferablypresent in a proportion of between about 0.3 and about 1% by weight. Bymaintaining the Ionol content of the fluid composition below 1.0%,preferably below 0.7%, and more preferably below 0.5% byweight,-toxicity of the composition is even lower than that of Skydrol®LD-4 hydraulic fluid.

As a copper corrosion inhibitor, the composition of the inventionpreferably includes a benzotriazole derivative, such as that sold underthe trade designation Petrolite 57068. This corrosion inhibitor ispresent in an amount sufficient to deactivate metal surfaces in contactwith the fluid composition against the formation of metal oxides on themetal surfaces in contact with the fluid, thereby reducing rates ofcopper dissolution into the hydraulic fluid, and also reducingdissolution of perhaps parts fabricated from copper alloys.Advantageously, the composition contains between about 0.005% and about0.09% by weight of the benzotriazole derivative, preferably betweenabout 0.02 and about 0.07% by weight.

Phosphate ester functional fluids are known to corrode iron alloys aswell as copper alloys. Numerous iron corrosion inhibitors are availablefor use in functional fluids, but these are known in many instances toincrease rates of erosion and thus have a net deleterious effect on theperformance properties of the hydraulic fluid. However, in accordancewith the invention, it has been discovered that certain4,5-dihydroimidazole compounds are effective iron corrosion inhibitors,yet do not adversely affect the erosion properties of the fluid. Useful4,5-dihydroimidazole compounds include those which correspond to thestructural formula

where R¹ is hydrogen, alkyl, alkenyl, hydroxyalkyl, hydroxyalkenyl,alkoxyalkyl or alkoxyalkenyl, and R² is alkyl, alkenyl or an aliphaticcarboxylate. Exemplary groups which may constitute R¹ include hydrogen,methyl, ethyl, propyl, butyl, pentyl, octyl, vinyl, propenyl, octenyl,hexenyl, hydroxyethyl, hydroxyhexyl, methoxypropyl, propoxyethyl,butoxypropenyl, etc. Exemplary group, which may constitute R² include,octyl, dodecyl, hexadecyl, heptadecenyl, or a fatty acid substituentsuch as 8-carboxyoctyl, 12-carboxydodecyl, 16-carboxyhexadecenyl, or18-carboxyoctadecyl. In a particularly effective embodiment, R¹ ishydrogen or lower alkyl and R² is a fatty acid residue containing atleast about 9 carbon atoms, i.e., —C₈—COOH to —C₁₈ COOH, preferablyC₁₆—C₁₈—COOH. In another preferred embodiment, R¹ is a lowerhydroxyalkyl and R² is a C₈—C₁₈ alkenyl. In the latter instance,however, the most satisfactory inhibition of Fe corrosion is realizedonly if the 4,5-dihydro-imidazole is used in combination with an aminoacid derivative, more particularly an N-substituted amino acid in whichthe N-substituent contains both polar and oleophilic moieties, forexample, an N-alkyl-N-oxo-alkenyl amino acid.

It has further and unexpectedly been discovered that the presence ofsuch a 4,5-dihydroimidazole compound, typically in a proportion ofbetween about 0.01% and about 0.1% by weight, not only inhibits ironcorrosion but contributes markedly to the stability of the functionalfluid as indicated by epoxide depletion.

It has been found that the salutary effect of the 4,5-dihydroimidazolecompound is enhanced if it is used in combination with a phenolicantioxidant, especially a complex hindered polyphenol such as abis(3,5-dialkyl-4-hydroxyaryl)methane or a1,3,5-trialkyl-2,4,6-tris(3,5-di-tert-butyl-4-hydroxyaryl)benzene.Exemplary of such complex hindered polyphenol compounds, respectively,are bis(3,5-di-tert-butyl-4-hydroxyphenyl)methane and1,3,5-trimethyl-2,4,6-tris(3,5-di-tert-butyl-4-hydroxyphenyl)benzene.Optimal effect on stability has been observed using a combination of thecondensation product of 4,5-dihydro-1H-imidazole and C₁₆-C₁₈ fatty acid(sold under the trade designation Vanlube RI-G by the Vanderbilt Co.)with a hindered polyphenol and an alkyl substituted diarylamine such asdi(p-octylphenyl)amine. Also effective as a 4,5-dihydroimidazolecompound in such combination is2-(8-heptadecenyl)-4,5-dihydro-1H-imidazole-1-ethanol (sold under thetrade designation Amine-O by Ciba-Geigy) To function as an ironcorrosion inhibitor, the latter compound should be used in combinationwith an amino acid derivative such as. e.g., theN-methyl-N-(1-oxo-9-octadecenyl)glycine sold under the trade designationSarkosyl®-O by Ciba-Geigy Corporation.

It has been found that a still further enhancement in high temperaturestability is realized where the 4,5-dihydroimidazole compound is used incombination with a phosphate ester base stock in which the alkylsubstituents attached to the phosphate moiety are substantially isoalkylC₄ or C₅ (namely, isobutyl or isopentyl).

Although they have not been found to produce the substantialadvantageous effect on high temperature stability that is afforded bythe use of an a 4,5-dihydroimidazole compound, other iron corrosioninhibitors have been found effective in the functional fluid of theinvention without adverse effect on erosion characteristics. Acceptableiron corrosion inhibitors include, for example, the product sold byPetrolite under the trade designation Petrolite P-31001.

As necessary, the fluid composition may also contain an anti-foamingagent. Preferably, this is a silicone fluid, more preferably apolyalkylsiloxane, for example, the polymethylsiloxane sold under thetrade designation DC 200 by Dow Corning. Preferably the anti-foam agentis included in a proportion sufficient to inhibit foam formation underthe test conditions of ASTM method 892. Typically, the anti-foam contentof the composition is at least about 0.0005% by weight, typically about0.0001% to about 0.001% by weight.

Preferably, the pH of the composition of the invention is at least about7.5, more preferably between about 7.5 and about 9.0. To impart a pH inthis range and to enhance the acid scavenging capacity of theformulation, the composition may further include between about 0.0035and about 0.10%, preferably between about 0.01% and about 0.1% byweight, most preferably between about 0.02% and about 0.07% of an alkalimetal phenate or other arenate. Potassium phenate is preferred. Inaddition to neutralizing acidic components of the composition, thealkali metal arenate serves to pacify the metal surfaces when thecomposition has been added to a hydraulic system, thereby reducingcorrosion.

Although optimal properties are realized in a composition of low alkyldiaryl phosphate content and particularly in compositions using the basestock of the invention as described above, the additive combination ofthe invention also affords beneficial results when used in combinationwith any of a variety of base stock compositions known to the art. Thebenefit of using esters whose alkyl substituents are predominantlycomprised of isoalkyl C₄ or C₅ (namely isobutyl or isopentyl) alsoextends beyond the preferred concentration ranges outlined above.Broadly, the additive combination can be used with an organophosphateester base stock comprising between about 10% and about 100%, preferablybetween about 10% and about 99%, by weight of a trialkyl phosphatewherein the alkyl substituents are substantially C₄ or C₅ (namely, butylor pentyl), preferably isoalkyl C₄ or C₅ (namely, isobutyl orisopentyl), between about 0% and about 70% by weight of a dialkyl arylphosphate wherein the alkyl substituents are substantially C₄ or C₅(namely, butyl or pentyl), preferably isoalkyl C₄ or C₅ (namely,isobutyl or isopentyl), and between about 0% and about 25% by weight ofan alkyl diaryl phosphate wherein the alkyl substituents aresubstantially C₄ or C₅ (namely, butyl or pentyl), preferably isoalkyl C₄or C₅ (namely, isobutyl or isopentyl). In a preferred embodiment, theadditive combination is used with a base stock comprising between about10% and about 100%, preferably between about 35% and about 99%, byweight of a tributyl or tripentyl phosphate, more preferably triisobutylor triisopentyl phosphate, between about 0% and about 35% by weight of adibutyl aryl or dipentyl aryl phosphate, more preferably diisobutyl arylor diisopentyl aryl phosphate, and between about 0% and about 20% byweight of a triaryl phosphate, and between about 0% and about 20% byweight of a triaryl phosphate. The additive combination is alsoeffective in combination with other ranges of base stock compositions asset forth below:

TABLE 1 Base Stock, Weight % Phosphate Ester I II III IV V Tri (C₄/C₅alkyl)¹ 10-72 10-25 50-72 80-99 50-72 Di (C₄/C₅ alkyl)¹ 18-70 45-7018-75 18-75 Aryl C₄/C₅ Alkyl¹ Diaryl  0-25  5-25  0-10  0-10 Triaryl 1-20  0-10 ¹In a preferred embodiment, the alkyl substituents areisoalkyl C₄ or C₅

As discussed hereinabove, optimal properties are achieved by combiningthe preferred isoalkyl C₄ or C₅ (namely, isobutyl or isopentyl)phosphate ester base stock with the additive combination of theinvention. However, significant benefits in lower toxicity, lowerdensity, hydrolytic stability, oxidative stability, and thermalstability are afforded by the use of the isoalkyl esters with otheradditive combinations as well, while, at the same time, maintaining alevel of seal integrity equivalent to that experienced by thecorresponding normal alkyl phosphate esters. In a preferred embodiment,the isoalkyl C₄ or C₅ phosphate ester base stock contains between about10% and 100%, preferably between about 50% and about 85%, by weight of atrialkyl phosphate wherein the alkyl substituents are substantiallyisoalkyl C₄ or C₅ (namely, isobutyl or isopentyl), between about 18% andabout 35% by weight of a dialkyl aryl phosphate wherein the alkylsubstituents are substantially isoalkyl C₄ or C₅ (namely, isobutyl orisopentyl), and between about 0% and about 10% by weight, preferablybetween about 0% and about 5% by weight, of an alkyl diaryl phosphatewherein the alkyl substituents are substantially isoalkyl C₄ or C₅(namely, isobutyl or isopentyl). However, the benefits of using theisoalkyl substituents are so substantial that they are realized to asignificant extent over a considerably broader range of composition.Generally, therefore, a base stock which utilizes isoalkyl esters maycomprise 100%, preferably between about 10% and about 90%, by weight ofa triisobutyl or triisopentyl phosphate, between about 0% and about 70%by weight of a diisobutyl or diisopentyl aryl phosphate and betweenabout 0% and about 25% by weight of an alkyl diaryl phosphate.Preferably, the alkyl substituent of the alkyl diaryl phosphate is alsoisobutyl or isopentyl, especially when the alkyl diaryl phosphatecontent exceeds about 5%. The aryl substituents of these esters aretypically phenyl, but may also be an alkyl-substituted phenyl(alkylphenyl) wherein the alkyl substituent is C₁ to C₉, preferably C₃to C₄. Nonlimiting examples of the alkyl-substituted phenyl substituentsinclude tolyl (also known as methylphenyl), ethylphenyl,isopropylphenyl, isobutylphenyl, tert-butylphenyl, and the like.

The isoalkyl base stock should be combined with an acid scavenger in anamount effective to neutralize phosphoric acid or phosphoric acidpartial esters formed in situ by hydrolysis of any of the phosphateesters of the base stock. The acid scavengers described above arepreferred but other acid scavengers known to the art may be used. Theisoalkyl based functional fluids should also contain an antierosionadditive in an amount effective to inhibit flow induced electrochemicalcorrosion of flow metering edges of hydraulic servo valves in hydraulicsystems. These fluids should also contain a viscosity index improver inan amount effective to cause the fluid composition to exhibit theviscosity index stated above. The composition should further include anantioxidant in an amount effective to inhibit oxidation of the fluidcomposition components in the presence of oxidizing agents. Preferably,the ant-erosion agent, viscosity index improver, and antioxidantcomposition are as described above, but the benefits of the use of anisoalkyl base stock are also realized with other additive combinationsknown to the art.

Methods known to those skilled in the art may be used for thepreparation of the compositions of the invention. For example, a basestock comprising the phosphate esters may be prepared by mixing in anagitated stainless steel vessel. Additives may then be blended into thebase stock in the same vessel. As noted above, the viscosity indeximprover is preferably added in the form of a solution in a phosphateester solvent.

At temperatures above 200° F., the more preferred functional fluidcompositions of the invention exhibit thermal, oxidative, and hydrolyticstability two to three times greater than that of Skydrol® LD-4hydraulic fluid as measured by the depletion of epoxide acid scavengeras a function of time. Superior stability is exhibited even in thepresence of halogen-containing compounds such as trichloroethane. When a4,5-dihydroimidazole compound is included, the extent of improvement iseven greater. As a result of the relatively low phenyl ester content,the composition of the invention has a density of less than one gram percc, typically between about 0.97 and about 0.99 grams per cc. This is adesirable feature from the standpoint of fuel burn (consumption) inaircraft.

Shear stability of the fluid composition also compares favorably withcommercially available aircraft hydraulic fluids. Thus, for example,after 500 hour exposure to an accelerated degradation test in a typicalaircraft hydraulic pump system, the viscosity of the composition at −65°drops only from 4000 to 2400. In part, this advantage is believed toresult from the narrower range of molecular weight of the viscosityindex improver. Exposure to shear conditions tends to degrade highermolecular weight viscosity index improvers, so that compositions inwhich the molecular weight of the viscosity index improver isdistributed over a broad range tend to suffer a greater loss ofeffectiveness over time due to breakdown of the higher molecular weightspecies.

In part due to the relatively low concentration of2,6-di-tert-butyl-p-cresol, the toxicity of the fluid composition in theinvention is very low. Where an isoalkyl ester base stock is used,toxicity is even lower.

The following examples illustrate the invention.

EXAMPLE 1

A hydraulic fluid having the composition set forth in Table 1 wasprepared by mixing at ambient temperature in a 50 gallon stainless steeltank agitated with a 25 horsepower agitator having an anchor typeimpeller. The phosphate ester components were introduced into the tankfirst and, after a 30 minute period of initial mixing, the otheradditives were added in the sequence indicated in Table 2.

TABLE 2 Basis: 100 Gram Basis: Batch 80 Gallon Batch Component GramsGrams/Pounds Tributyl Phosphate, 49.0135 148,216.8/326.8 Neat DibutylPhenyl 26.43 79,652.2/175.6DRUM Phosphate of      2(−220#) Low DiphenylContent (Less Than 2% By Weight) Methacrylate Ester 16.56 50,077/110.4Viscosity Index    22684.9 Improver (PA6477,    gSLDS 45.3% solids in54.7% tributyl phosphate) 3,4 Epoxycyclohexane 6.3 19,051/42 CarboxylateFC98 [Potassium .05 151.2/ Perfluorooctylsulfonate (FC98)] Benzotriazoletype .05 151.2/ Copper Corrosion Inhibitor (P57068, Petrolite (50%Active), EX1663 Iron Corrosion Inhibitor .05 151.2/ (90-31001, Petrolite(50% Active) Dye .001 3.204/ Potassium Phenate .035 105.84/Bis-(3,5-di-tert-butyl 0.90 2,722/6 4-hydroxyphenyl) methane (Ethanox  ®702) Di(p-octylphenyl)amine 0.45 1,361/3 2,6-Di-tert-butyl-p-cresol 0.25756/1.667 Antifoam 0.0005 1.512/ (Dow-Corning)

This composition had a density of 0.996 g/cc at a temperature of 25° C.Of the source of dibutyl phenyl phosphate, 77.135% by weight was dibutylphenyl phosphate or butyl diphenyl phosphate, so that 20.3% by weight ofthe overall composition was constituted of phosphate esters containing aphenyl moiety. However, the butyl diphenyl phosphate content was lessthan 1% by weight. Triphenyl phosphate content was essentially nil.

EXAMPLE 2

A second aircraft hydraulic fluid composition was prepared in the mannergenerally described in Example 1. The composition of this fluid is setforth in Table 3.

TABLE 3 Basis: 100 Gram Basis: 80 Gallon Batch Batch Component GramsGrams/Pounds Tributyl Phosphate 50.5988 152,999.3/337.3 Dibutyl Phenyl24.0947 72,862.3/106.63 Phosphate of Low Diphenyl Content (Less Than 2%By Weight) Methacrylate Ester      22,684.9 Viscosity Index      gSLDSImprover (PA6477,      Total 43.8% solids/56.2% tributyl phosphate) 3,4Epoxycyclohexane 6.3 19,051/42 Carboxylate FC98Potassium .05 151.2/Perfluorooctylsulfonate (FC98) Benzotriazole Type .05 151.2/ CopperCorrosion Inhibitor (P57068, Petrolite; (50% Active) Iron CorrosionInhibitor .05 151.2/ (90-31001, Petrolite (50% Active), EX1663 Dye .0013.024/ Potassium Phenate .035 105.84/ Bis-(3,5-di-tert-butyl 0.902,722/6 4-hydroxyphenyl) methane (Ethanox  ® 702) Di(p-octylphenyl)amine.45 1,361/3 Dow-Corning .0005 1,512/ Antifoam 2,6-Di-tert-butyl-p-cresol.25 756/1.667

This composition also exhibited a density of 0.996 g/cc at a temperatureof 25° C. Of the source of dibutyl phenyl phosphate, 84.751% by weightwas constituted of esters which contained no phenyl moiety. The overallcomposition contained 20.3% by weight of phosphate esters having aphenyl moiety, but less than 1% by weight butyl diphenyl phosphate andessentially no triphenyl phosphate.

Set forth in Table 4 are a partial elemental analysis and measuredphysical properties of the compositions of Examples 1 and 2. These dataestablish that the fluid composition of Examples 1 and 2 meet or exceedthe airframe manufacturers' specification, for properties needed toqualify a product for use as an aircraft hydraulic fluid.

TABLE 4 BATCH 1 BATCH 2 COLOR PASS PASS CHLORINE, PPM 20 21 K+ 106 99 S57 83 Ca <1 <1 Na 1.4 1.5 SP.G. .9972 .9975 VISC. 210 F. CST 4.75 4.81100 13.65 13.91 −65 1635 1628 MOISTURE .10 .12 NEUT NO .01 .02 POUR PT.° F. <−80 <−80 AIT. F 850 920 FLASH PT. 350 360 FIRE PT. 360 390CONDUCTIVITY .65 .55 OXIRANE NO. .39 .40 FOAM SEQ 1 170/65 180/20 2 30/10  40/44 3  80/35 140/56 PARTICLE COUNT 5-15 7247 3116 15-25 1444513 25-50 460 180 50-100 75 53 >100 14 10 SILTING INDEX 1.18 1.05

EXAMPLE 3

Tests were conducted comparing the thermal, oxidative and hydrolyticstability of the hydraulic fluid compositions of Examples 1 and 2 withcommercially available hydraulic fluids. In each of these tests, a 301stainless steel tube was filled to 80% capacity with the fluid to betested. The temperature was maintained constant in each test.Compartative tests were run at 50° F. and 275° F., and further tests ofthe composition of the invention were run at 300° F. In all tests, fivecorrosion coupons were immersed in the fluid.

In some of the tests, the head space in the tube was filled with air, inothers it was filled with nitrogen. After each tube was filled with theappropriate test composition, it was capped and heated to apredetermined test temperature and maintained at that temperature sothat hydrolytic stability at such temperature could be determined. Eachtube was monitored over time and samples were taken to follow trends inthe fluid's chemical composition, in particular the concentration of theacid scavenger (epoxide) present in the sample. When the epoxide is 100%depleted, the fluid is typically degraded to the point that itsusefulness as an aircraft hydraulic fluid has essentially beenexhausted. As epoxide depletion approached 100%, test specimens weretitrated for acidity. When the neutralization number of the fluidreached 1.5 or greater, the test was halted.

Illustrated in FIGS. 1 to 3 are epoxide depletion curves for thecompositions of the invention as compared to previously availableaircraft hydraulic fluids. In these curves, and in those relating to thefurther examples set forth below, the legends “W17” and “W17R” designatea composition of Table 1 or 2 above. “2495B1” refers specifically to thecomposition of Table 1, and “2495B2” to the composition of Table 2.“H4A” refers to commercial hydraulic fluid sold by Chevron under thetrade designation “Hyjet IVA®.” “Epox A” means that the test was runwith air in the head space of the stainless steel tube, so that the testspecimen was exposed to thermal, hydrolytic, and oxidative effects.“Epox T” means that the head space contained nitrogen, so that the testprimarily measured thermal hydrolytic effects only.

EXAMPLE 4

Further thermal, hydrolytic, and oxidative stability tests wereconducted on the compositions of Example 1 and 2. These tests werecarried out generally in the manner described in Example 3, except that0.5% moisture was incorporated in the test samples to determine theeffect of moisture on thermal stability. Test temperatures were 250° F.and 275° F. The results of these tests are plotted in FIGS. 4 and 5.

EXAMPLE 5

Additional thermal, oxidative, and hydrolytic stability tests comparingthe compositions of the invention with those previously available in theart were conducted in sealed pyrex tubes. In certain of the tests,corrosion coupons were immersed in the liquid contained in the pyrextube. Except for the use of pyrex rather than stainless steel tubes, thetests were conducted in essentially the manner described in Example 3.Both the compositions of the invention and comparative fluids weretested at 300° F. in the presence of 0.1 to 0.5% moisture with fivecorrosion coupons immersed in the test samples. The results of thesetests are set forth in FIGS. 6 to 8. Additional tests on thecompositions of the invention were conducted at 375° F. without moistureaddition. The results of these tests are set forth in FIG. 9.

EXAMPLE 6

Further thermal, oxidative, and hydrolytic stability tests wereconducted generally in the manner described in Example 3, except thattrichloroethane was added, in varying amounts, to the test specimens inorder to determine the effect on stability. Test temperatures were 275°F. and 300° F. The results of the tests of this example are set forth inFIGS. 10 and 11.

EXAMPLE 7

The oxidation and corrosion resistance of the fluid compositions ofExamples 1 and 2 was compared with that of previously available aircrafthydraulic fluids by testing in accordance with federal test methodFTM5308.7 This test severely stresses the fluid with regard to oxidationstability.

In each test the fluid was charged to a glass tube and tested inaccordance with FTM 5308.7. The fluid was heated to a fixed temperatureof 350° F. after which dried air was purged through the test fluid at arate of 5 liters per hour. Samples were taken every 24 hours, or morefrequently, and the test was halted when the neutralization number ofthe fluid reached 1.5 or greater. The results of the tests in thisExample are illustrated in FIG. 12.

EXAMPLE 8

Because erosion is a form of electrochemical corrosion, erosioncharacteristics of a hydraulic fluid composition can be measured by wallcurrents obtained during flow of the fluid through small simulatedorifices similar to those in a test servo valve. Using a standarderosion test apparatus, tests were conducted comparing the erosionproperties of the compositions of Examples 1 and 2 with aircrafthydraulic fluid compositions previously available to the art. In thistest system, favorable erosion properties were indicated by low wallcurrents and the most favorable characteristics are indicated by anegative wall current. Set forth in Table 5 is a summary of the dataobtained in testing the compositions of the invention and thosepreviously available commercially.

Further erosion tests were conducted on various functional fluidcompositions after storage in glass containers at contact with air at225° F. Set forth in Table 6 are the results of these tests for samplesstored for the indicated number of hours.

In these tables, two measurements are reported for conductivity of thespecimen, one taken by applicant's assignee and the other by an outsidetesting laboratory. I_(w) designates wall current, i_(t) designatesthreshold current, and R_(v) is the rate of erosion. R_(v) is related toI_(w) and i_(t) by the function:

R_(v)=150I_(w)−18i_(t)

In Tables 5 and 6, the term “LD4” refers to the product sold under thetrademark “Skydrol® LD-4” by Monsanto; “SKY500B” and “B4” refer toanother functional fluid product available from Monsanto under the tradedesignation of “Skydrol® 500B4”; “LD5” refers to the composition of theinvention; “FC_(96”)“FC₉₆ ” refers to an antierosion agent comprising apotassium salt of perfluorohexylsulfonic acid; “Ca+2” refers to thepresence of Ca⁺² di(perfluoromethylsulfonate) in a tested fluid; “AO”means that an antioxidant was present, typically a combination of Ionoland a hindered polyphenol such asbis(3,5-di-t-butylhydroxyphenyl)methane; “X1” with reference to theantierosion in LD4 means that the antierosion agent FC₉₈ is present inthe standard commercial concentration; “X2” and “X3” meant that the FC₉₈concentration has been doubled or tripled; “TBP” refers to tributylphosphate; “DBPP” refers to dibutyl phenyl phosphate; “TEHP” refers totriethylhexyl phosphate; “Si—HC” refers to a tetraalkyl silanecomposition; “HT” is used to designate Skydrol® HT, a functional fluidformulation that has been sold by applicant's assignee; “TiBP” refers totriisobutyl phosphate; “FC98” refers to an antierosion agent comprisinga potassium salt of perfluorooctylsulfonic acid a mixture of a potassiumsalt of perfluoroethyl cyclohexyl sulfonate, a potassium salt ofperfluoromethyl cyclohexyl sulfonate, a potassium salt ofperfluorodimethyl cyclohexyl sulfonate, and a potassium salt ofperfluorocyclohexyl sulfonate; “EXI 663” refers to a benzotriazole Cucorrosion inhibitor; 31001 refers to a Petrolite Fe corrosion inhibitor;HALS refers to a hindered amine light stabilizer; “H4A” refers tovarious samples of the functional fluid sold commercially by Chevronunder the trade designation Hyjet IVA; “W6”, “W7”, “W8,” etc. refer tothe compositions of the invention; “ERT” means the specimen had beenused in Erosion Resistance Tests; and “ECT” means the specimen had beenused in Erosion Control Tests.

TABLE 5 EROSION TEST DATA SUMMARY Independent Lab Cond. MCC Cond I_(w)I_(l) R_(v) Sample ID μMHO/cm μMHO/cm μA μA/cm² cm³/min/h LD4, Duplicate8/88 0.370 0.410 0.036 2.650 −12.000 LD4 0.360 0.350 0.046 1.200 −15.000LD4, W/FC98X2 0.640 0.620 0.012 6.000 −106.000 LD4[FC96.250 PPM] 0.2400.320 0.110 0.310 11.000 LD4[FC96.1250 PPM] 0.780 0.810 0.089 2.350−28.000 LD4[FC96.2500 PPM] 1.200 1.220 0.061 4.100 −65.000 LD4[FC98.73PPM] 0.190 0.240 0.086 0.700 0.000 LD4[500 PPM, Ca + 2.] 0.670 0.750−0.005 13.000 −235.000 LD4[1000 PPM Ca + 2.] 0.980 0.940 −0.003 <18.000NEG LD4[1500 PPM, Ca + 2.] 1.200 1.150 −0.003 <19.000 NEG HY JET IV1.000 −0.034 1.850 −40.000 B4[500B4] 0.300 0.019 1.150 −18.000 TBP 0.0080.450 0.038 67.000 DBPP 0.008 0.460 0.094 67.000 TEHP 0.001 0.021 — <3Si-HC 0.000 <0.0001 — <.0015 HT, FC98 0.037 0.690 0.410 0.210 58.000 HT,FC98X1 0.630 1.020 0.000 2.850 −54.000 TiBP 0.001 NV 0.127 0.007 20.000LD5[FC98.250 PPM] 0.150 0.071 1.250 −12.000 LD5[FC98.750 PPM] 0.2500.015 1.400 −23.000 LD5[FC98.250 PPM] 0.140 −0.017 −0.061 NEG .02% H₂OLD5[SAME] .1% H₂O 0.150 −0.007 −0.375 NEG LD5[SAME] .2% H₂O 0.150 −0.055−0.375 NEG LD5[SAME] .3% H₂O 0.160 −0.085 −0.400 NEG LD4, .1% H₂O [.51%]0.370 0.001 2.000 LD4, .2% H₂O [.45%] 0.380 0.007 1.700 LD4, .3% H₂O[.56%] 0.400 0.014 1.800 H4A 0.930 0.096 16.450 −262.000 H4, Used 0.3000.011 2.300 −40.000 LD4, Used 0.390 −0.053 0.990 −26.000 SKY500A 0.0390.185 0.600 16.950 H4A#2, Used 0.450 0.010 3.400 −59.700 H4A#1, Used0.510 0.010 2.800 −48.900 H4A#3, Used 0.670 0.020 2.400 −40.200 LD4,Ca[S03C4F9]2 0.570 −0.020 >11 −201.000 H4A#5, Used + Cl 0.670 0.0202.400 −40.000 H4A#5, Used @ 195 H, 0.770 0.050 7.400 −126.000 NO ClH4A#5, Used + Cl .500 H 0.440 0.026 0.260 −1.000 W6, Fresh 0.620 0.6300.150 1.600 −6.000 Used-600 h. 0.630 0.740 −0.021 1.600 −55.000 W7,Fresh 0.490 0.590 0.140 1.300 −2.000 Used-600 h. 0.610 0.760 −0.0132.200 −42.000 W8, Fresh 0.580 0.560 0.167 0.580 0.140 Used-600 h. 0.8800.760 0.004 2.250 −40.000 W9, Fresh 0.540 0.640 0.230 1.400 11.000 W10,Fresh 0.310 0.380 0.230 1.400 9.000 Used-600 h. 0.730 0.750 0.036 2.000−31.000 W11, Fresh 0.500 0.580 0.240 1.080 17.000 W12, Fresh 0.560 0.5900.160 3.200 −34.000 Used-600 h. 0.670 0.690 0.160 2.500 −18.000 W13,Fresh 0.670 0.690 0.160 2.500 −21.000 Used-600 h. 0.970 1.000 0.0012.750 −48.000 W14, Fresh 0.52 0.55 0.17 9.10 −138.00 Used-600 h. 0.670.73 −0.01 1.75 −33.00 W15, Fresh 0.51 0.54 0.16 0.63 13.00 Used-600 h.0.62 0.75 −0.02 1.60 −31.00 W15, Fresh, Erosion 0.07 0.54 −0.01 1.75−33.00 Control Used-600 h. 0.75 W16, Fresh 0.670 W17, Fresh, Abex + 2000.580 0.180 1.200 8.500 PPM Cl Used-600 h. 0.560 −0.028 0.720 −17.000Used, ERT 0.610 −0.016 LT. .29 GT. −8 Used, ECT 0.66 −0.04 LT. .35 GT.−13 LD4 + FC910 0.230 0.026 1.500 −23.000 LD4 + H4A-AO'S 0.350 0.0320.920 −12.000 LD4 + HALS, NO-FC98 0.015 0.120 0.140 16.000 LD4 + HALS +FC98 0.410 0.071 0.490 0.200 LD5, W17.2X − FC98 0.420 0.110 2.250−24.000 ONLY NBP4419198 +50 PPM EX1663 0.430 0.100 1.060 −4.000 +250 PPMEX1663 0.430 0.110 1.070 −3.000 +1000 PPM EX1663 0.450 0.120 1.120−3.000 +50 PPM 31001 0.420 0.120 1.060 −1.000 +250 PPM 31001 0.430 0.1001.080 −4.000 +5 PPM KP 0.450 0.170 0.510 16.000 +350 KP 0.700 0.2100.800 17.000 +500 PPM DODPA 0.430 0.120 1.080 −1.000 +5000 PPM DODPA0.420 0.120 1.080 −1.000 LD5, W17, NO AEA 0.023 0.230 LT. .01 GT. 34NBP4419199 + 160 PPM 0.31 −0.01 GT. 12 LT. −220 Ca(SO₃C4F9)₂

TABLE 6 Erosion Test Data After Oven Heating 225 F. In Glass; Air @Start Only; Includes 1020 Steel and Cu Corr. Coupons Independent MCC LabCond. Cond. I_(w) I_(l) Rv μmHo/cm. μmHO/cm μA μA m3/min/h. LD-4 Hours100.000 0.390 0.039 2.500 −39.000 200.00 0.410 −0.009 0.200 −5.000300.00 0.410 −0.001 0.170 −4.000 600.00 0.360 0.012 0.410 −6.000 H4AHours 100.000 1.200 0.087 0.097 11.000 200.000 1.100 0.083 0.330 6.000300.00 1.000 0.088 0.280 8.000 600.00 1.100 0.086 0.350 7.000

EXAMPLE 9

The compositions of Examples 1 and 2 were compared with an availablecommercial hydraulic fluid in a storage test at 375° F. in the presenceof iron. After 21 hours storage at such conditions, analyses were madeof the solids build-up in the fluid. More particularly, measurementswere made of the build-up of metal solids, other solids, and totalsolids. The results of these tests are illustrated in FIG. 13.

EXAMPLE 10

Aircraft hydraulic fluids of the invention were formulated,substantially in the manner described in Example 1, and subjected to theErosion Resistance Test of Boeing Material Specification for FireResistant Hydraulic Fluid. BMS 3-11G (Rev. Jul. 17, 1986). Set forth inTables 7, 7A, and 7B are the compositions of the fluids tested. Setforth in Table 8 are the results of the erosion tests. Set forth inTables 9 and 9A is a comparison of the properties of the fluids beforeand after subjection to the erosion tests. In these tables. “HF 400,”“HF411,” and “HF-460” refer to poly(butyl/hexyl methacrylate) viscosityindex improvers. In each entry, the table states the butyl methacrylatepolymer solids content, the balance being trialkyl phosphate solvent.“AEA” refers to an antierosion agent, “PANA” designatesphenyl-α-napthylamine; “APANA” designates analkylphenyl-α-naphthylamine. “DODPA” refers to di(p-octylphenyl)amine;“P58526 Petrolite” is an iron corrosion inhibitor, “DC 200, 100 CST” isa Dow-Corning antifoam; “SARK O” refers to theN-methyl-N-1-OXO-9-octadenyl) glycine sold under the trade designation“Sarkosyl-O” by Ciba-Geigy; “AMINE O” refers to the2-(8-heptadecenyl)-4,5-dihydro-1H-imidazole-1-ethanol sold under thetrade designation “Amino-O” by Ciba-Geigy; “90-31001” refers toPetrolite 31001; and “FH-132” refers to diphenyldithioethane.

TABLE 7 FORMULATIONS VARIABLE W-1 W-2 W-3 W-4 W-5 TiBp 54.29^(a)53.33^(a) 54.58^(a) 52.61^(a) 39.8653^(a) DiBPP, 66.3% PH 29.90^(b)29.92^(b) 29.90^(b) 29.88^(b) 26.45^(b) PA6385 8.52 8.47 8.21 — — PA6703— — — 10.16 10.16 MCS 1562 6.3 6.3 6.3 6.3 6.3 AEA, FC98 .05 .05 .05 .05.05 P57068, PET (50% .05 .05 .05 .05 .05 ACTIVE) DYE .00 .001 .001 .001.001 KP .03 .035 .035 .035 .035 E702 .90 — .45 .45 .9 DODPA .45 .45 .15.45 .45 IONOL .25 — .25 .25 .25 DC 200, 100 .005 .0005 .0005 .0005 .0005CST VANLUBE — — .025 .025 0.025 RI-G L130 1. — — — — E330 .3 1.05 — — —L57 .4 — — — — E703 .3 .35 — — — FORMULATIONS W15A W17 W18 TBP 39.865349.3685 39.8653 DBPP, LOW DI-PHENYL, ROD/C2 35.76 26.45 26.45 (D56.8P)DBPP, LOW DI-PHENYL, ROD/C4 HF400, 43.6% S/7.5% FINAL 17.36 17.36 HF411,35.5% s/3.75% FINAL 6.41 HF460, 58.5% s/3.75% FINAL 10.42 MCS 1562 5.86.3 6.3 AEA, FC98 .05 .05 .05 P57068, PETROLITE (50% ACTIVE) .04 .05 .05EX1-663 DYE .001 .001 .001 KP .035 .035 .035 E702 .76 .9 .9 DODPA . .45.45 IONOL . .25 .25 A-PANA .85 . . P58528, PETROLITE (50% ACTIVE) . .05.05 90-31001 DC 200, 100 CST .0005 .0005 .0005 SARK O .004 . . AMINE O.004 . . FH132 .25 ^(a)Triisobutyl phosphate ^(b)Diisobutyl phenylphosphate

TABLE 7A FORMULATIONS VARIABLES W6 W7 W8 TBP, REDIST. 45.835 . . TBP .50.844 50.8935 DBPP, LOW DI-PHENYL, ROD/C2 30. 25. 25. DBPP, LOWDI-PHENYL, ROD/C4 . . . HF400, 43.6% S/7.5% FINAL . . . HF411, 35.5%S/3.75% FINAL 10.42 10.275 10.275 HF460, 58.5% S/3.75% FINAL 6.41 6.416.41 MCS 1562 5.8 5.8 5.8 AEA, FC98 .05 .05 .05 P57068, PETROLITE .055.1 .1 (50% ACTIVE) DYE .001 .001 .001 KP .035 .035 .01 E702 .761 . .PANA .625 . . APANA . .76 .9 DODPA . .625 .45 P58528, PETROLITE . .1 .1(50% ACTIVE) DC 200, 100 CST . . .0005 SARK O .004 . . AMINE O .004 . .(1)KP, SELFMADE KP 2% BDPP IN DBPP

TABLE 7B FORMULATION VARIABLES W15 TBP 39.8653 DBPP, LOW DI-PHENYL,ROD/C2 35.76(D/56.8P) DBPP, LOW DI-PHENYL, ROD/C4 HF400, 43.6% s/7.5%FINAL HF411, 35.5% s/3.75% FINAL 6.41 HF460, 58.5% s/3.75% FINAL 10.42MCS 1562 5.8 AEA, FC98 .05 P57068, PETROLITE (50% ACTIVE) .04 EX1-663DYE .001 KP .035 E702 E702.76 PANA .85 P58528, PETROLITE (50% ACTIVE)90-31001 DC 200, 100 CST .0005 SARK O .004 AMINE O .004

TABLE 8 RUN NUMBER 3 4 5 6 7 8 RIG USED A A A C A C CASE DRAINTEMPERATURE (° F.) 290 290 290 315 315 315 RESERVOIR TEMPERATURE 275 275275 300 300 300 Cl ADDED, PPM 0 0 0 0 0 0 TOTAL RUN TIME, HR 468 368 570560 475 420 OPERATING PROBLEMS O rings O rings shut none pump pump downswater water BOEING VALVE DATA SLIDE AND SLEEVE NO. W004 W004 W002 W011W008 W011 PORT NUMBERS 5.7 6.8 6.8 1.3 1.3 2.4 FLOW INCREASE, cc/min.erratic erratic erratic 40 200 200 ACCEPTABLE? no no no yes marginalmarginal EDGE APPEARANCE slight slight slight shaded slight slight wearwear wear wear wear PUMP DATA MANUFACTURER Vickers Vickers Vickers AbexVickers Abex SERIAL NO. 491761 491761 491761 166495 482891 166495 HRS ATSTART 0 468 856 1000 0 1562 @225 F. HRS TO FALURE 468 856 1426 no 4761980 failure CAUSE OF FAILURE O ring O ring bearings — bearings bearingsshaft seal SECOND PUMP (IF USED) MFR S/N HRS AT START HRS TO FAILURE 1515 15 RUN NUMBER ERT ECT ERT RIG USED C A C CASE DRAIN TEMPERATURE (°F.) 315 275 300 RESERVOIR TEMPERATURE 300 280 284 Cl ADDED, PPM 10001000 200 TOTAL RUN TIME, HR 274 245 500 OPERATING PROBLEMS none noneshaft seal BOEING VALVE DATA SLIDE AND SLEEVE NO. W007 W022 W007 PORTNUMBERS 6.8 5.7 1.3 FLOW INCREASE, cc/min. extreme extreme 300ACCEPTABLE? no no marginal EDGE APPEARANCE severe severe slight wearwear shading PUMP DATA MANUFACTURER Abex Vickers Abex SERIAL NO. 183629491761 115815 HRS AT START 0 0 0 HRS TO FALURE 274 230 320 CAUSE OFFAILURE Ou Ou shaft transfer transfer seal SECOND PUMP (IF USED) MFRVickers Abex S/N 491763 228188 HRS AT START — 0 492891 both destroyedHRS TO FAILURE 130 17 17 18 17 17 BASE 17 17 BMS 18 BASE RUN NUMBER ERTECT CASE AIRBUS @225 F. PUMPING ERT CASE RIG USED A A C C B HP B A CASEDRAIN TEMPERATURE (° F.) 300 275 300 290 240 284 300 300 RESERVOIRTEMPERATURE 284 260 284 273 225 235 284 284 Cl ADDED, PPM 200 400 0 01000 0 200 0 TOTAL RUN TIME, HR 500 330 800 1000 1000 500 500 760OPERATING PROBLEMS none none pump none none none none none water BOEINGVALVE DATA SLIDE AND SLEEVE NO. W006 W022 W020 W020 W017 not used W017W016 PORT NUMBERS 2.4 1.3 1.3 5.7 6.8 — 2.4 2.4 FLOW INCREASE, cc/min.100 500 0 40 170 — 40 136 ACCEPTABLE? yes yes yes yes yes — yes yes EDGEAPPEARANCE slight worn slight slight slight — slight slight shadingshading wear wear wear wear PUMP DATA MANUFACTURER Vickers Vickers AbexAbex Vickers Abex Vickers Vickers SERIAL NO. 491761 491763 183629 226153492891 L-1976 491762 491761 HRS AT START 0 0 0 0 0 0 0 0 HRS TO FALUREno no 676 1000 no no no no failure failure failure failure failurefailure CAUSE OF FAILURE — — bearings bearings — — — — SECOND PUMP (IFUSED) MFR Abex S/N 116815 HRS AT START 0 HRS TO FAILURE 117

TABLE 9 SOME DATA FROM THE ANAL. FLUIDS: MCS2510- WLFR WLU W2.FR W2.UW3.FR W3.U W4.FR W4.U W5.FR W5.U (@500 HRS) SP. GR. .9868 .9925 .9877.9890 .9896 .9845 .9902 .9898 .9892 .9905 VISC 210 3.65 2.54 3.69 2.193.32 2.19 4.2 4.28 2.99 2.49 100 11.47 9.05 11.94 7.24 10.47 6.93 12.978.46 8.99 7.96 −65 3954 5754 4963 3302 3632 2685 3893 2158 2317 2421 NN.02 ND .01 1.05 .01 .04 .03 1.86 .01 .09 % H₂O .12 .04 .13 .11 .08 .05.11 .02 .15 .07 AlT 930 930 94 0930 940 920 960 950 930 940 FL. PT 330265 310 290 315 300 350 350 335 319 Fl. PT. 350 335 340 330 355 350 370390 365 381 OX OX. TD TD .41 TD .38 TD .39 .14 .61 .36 COND. .44 — .28.90 .45 .37 .36 1.63 .41 .43 −% EPOX — 86.5 — 65.3 — 22.1 — 78.9 — 57.8Cl 15 154 25 173 12 257 7 204 18 136 HRS 580 — 502 — 579 — 334 — 933 —TEMP. F. 290/284 — 293/284 — 297/290 — 278/270 — 300/280 — PUMP RIG A —— A — B — A — C AEA FC98 2XSTD — 2XSTD — 2XSTD — 2XSTD — 2XSTD — ICAPDATA: WLFR WLU W2.FR W2.U W3.FR W3.U W4.FR W4.U W5.FR W5.U Na 5.23 28.32.5 8.1 4.7 8.8 3.2 14.9 3.9 8.7 K 74.6 87. 71.1 64.3 91.1 49.4 104.594.6 110.3 34.9 S 58.4 56.8 58.3 59.1 61.5 73. 79.3 79.5 63.9 73.8 Cu1.32 720 <.125 142.1 1.1 .8 <.13 1112 <.13 6.7 Fe <.5 134.9 <.125 11.9<.25 <.5 <.13 140.3 <.13 1.3 Mn <.5 1.53 <.125 <.5 <.25 <.5 <.13 .86<.13 <.5 Zn <.5 93.9 <.125 14.28 <.25 <.5 <.13 131. <.13 .9 Al <.5 2.11<.125 <.5 <.59 <.5 <.13 <.5 <.41 <.5 Cd <.5 7.25 <.125 1.54 <.25 <.5<.13 6.62 <.13 <.5 FOAM 35/23 ND ND ND ND 40/19 ND 160/91 80/34 50/21TEST (250/100)F (400/250U) W6.FRESH .−USED W7.FRESH .−USED W8.FRESH.−USED SP. GR. 1.0015 1.0048 .9991 1.0003 .9993 .9995 VISC. 210 4.623.02 4.73 2.38 4.80 2.70 100 13.48 9.28 13.59 7.09 13.80 7.98 −65 15231181 1456 776 1471 809 NN .02 1.09 .13 .14 .14 .212 % H₂O .07 .04 .11.02 .16 .015 AlT 910 950 870 925 900 9700 FL. PT 320 315 320 300 3303100 Fl. PT 360 365 360 350 375 3500 OX. OX. .39 ND .41 .14 .40 .118COND. .63 .74 .59 .76 .56 .768 −% EPOX 0 85.6 0 57.6 0 69.4 Cl — 11 1110 15 10 HRS — 124/438 — 475 — 418 TEMP. F. — 275/300 — 300 — 300 AEA.FC98 2XSTD — 2X STD 2X STD — PUMP RIG C — A− — C — O&C LIFE 72 — 120 —120 SPAN @ 350 F., HRS ICAP DATA: W6/F /U: W7/F /U: W8/F /U: Na <.5 2.58<.5 2.07 <.5 3.1 K 112.7 54.95 98.06 83.54 72.58 49.15 S 96.53 103.294.97 144.5 81.94 75.17 Cu <.5 1.13 <.5 23.5 <.5 3.37 Fe <.5 <.5 <.5 <.5<.5 1.63 Zn <.5 1.42 <.5 6.96 <.5 17.94 Al 1.17 1.11 <.54 <.5 <.5 <.5 Cd<.5 <.5 <.5 .54 <.5 <.5 FOAM TEST 500/1500, 35/15; 320/>600, 20/6;180/83, 60/19 (250/100)F −INCR. IN 350 − 300 = 50 600 − 410 = 190 600 −300 = 300 INT. LEAKAGE CC'S/MIN 0 HR · RUN END 350 − 320 = 30 600 − 390= 210 600 − 500 = 100 200< <500 HRS. EROSION DE-ALLOYED DE-ALLOYEDDE-ALLOYED TYPE VIA:SEM X ND ND ;VISUAL X X X EROSIVE.PUMP — NO — YES(1)— YES(1) .BECK NO NO NO NO YES NO (1)LESS EROSION THAN H4A AT 225 F. FOR600 HRS. SOME DATA FROM THE ANAL. FLUIDS: W15 USED USED W17 USED USEDW17 USED FRESH ECT ERT FRESH B1 B2 ERT ECT ERT SP. GR. .9996 .9992 .9992.9990 .9978 .9976 .9990 1.0314 1.0005 VISC. 210 5.23 2.68 2.68 4.94 4.974.91 2.74 2.54 2.46 100 15.19 7.87 7.87 14.12 14.43 14.28 8.26 7.77 7.27−65 1576 779 799 1426 1777 1719 1024 1289 769 mn .03 .66 .64 .02 .02 .02.06 .3 .05 % H₂O .14 .02 .02 .15 .14 .10 <.01 <.01 .02 AlT 890 910 NA840 NA NA 870 970 NA FL. PT 330 305 315 350 330 325 320 340 315 Fl. PT.375 350 350 385 365 365 355 380 350 OX. OX. .38 .02 .09 .46 .41 .41 <.01<.01 .11 COND. .54 .75 .71 .54 .66 .66 .73 .8 .68 −% EPOX 0 82.1 65.2 00 0 57.3 80 56.4 Cl 11 1290 1334 7 5 4.4 263 237 183 HRS 0 245 264 0 0 0498 342 418 TEMP. F. — 262 315 — — — 300/284 275/262 300/284 AO'S. RAT.7 .9E702 APANA .9 .45 .25− — — — AEA FC98 2XSTD — — 2XSTD — — — — — NEWAEA NO — NO NO NO NO NO NO — −INCR. IN <600 INT. LEAKAGE CC'S/MIN 0 HR ·RUN END 200< <500 HRS EROSION TYPE VIA:SEM ;VISUAL EROSIVE.PUMP — YESYES .BECK — YES PUMP RIG C — C C — — B A C#1 O&c LIFE — — — — — — — — —SPAN @ 350 F., HRS 120 NA ICAP DATA: W15/F W15/U W15/U W17/F B1 B2 W17/UW17/U W17/.U Na .94317 3.063 .606 .56 <.5 <.5 2.05 2.39 <.5 K 84.14601.9 46.99 76.15 82.4 85.1 35 45.5 46.62 S 79.39 64.14 87.5 59.11 63.961.9 60.6 561.7 69.2 Cu <.5 1213 9.811 <.5 <.5 <.5 9.32 95.76 11.34 Fe<.5 43.53 293.3 <.5 <.5 <.5 8.24 60.89 50.4 Mn <.5 .435 1.775 <.5 <.5<.5 <.5 <.5 <.5 Zn <.5 <.5 58.02 <.5 1.76 2.16 1.09 13.22 14.17 Al .942.475 27.2 1.59 <.5 <.5 <.5 <.5 <.5 Cd <.5 <.5 <.5 <.5 <.5 <.5 <.5 <.5<.5 W15/F W15/U W17/F W17/U W17/U W17/U FOAM 280/170 440/268 NA 210/9370/25 55/18 60/20 TEST 240/130 (250/100)F (400/250)U

EXAMPLE 11

Formulations were prepared which substantially corresponded to thecompositions of Example 1, except that the trialkyl phosphate anddialkyl aryl phosphate components were triisobutyl phosphate anddiisobutyl phenyl phosphate, respectively, and the compositions variedwith respect to the compound included as an iron corrosion inhibitor.Erosion valve leakage tests were run on these compositions in the mannerdescribed in Example 9, and epoxide depletion tests were conducted onthese compositions generally in the manner described in Example 1. Theresults of these tests are set forth in Table 10.

The table indicates that composition M-1 used a “combination” ofantioxidants. Initially, M-1 contained Ionol, Ethanox 702 anddi(p-octylphenyl)amine (DODPA). After the erosion test had progressedfor 25 hours, further amounts of Ethanox 702 and DODPA were added to thecomposition. At 153 hours, a phenolic antioxidant was added; at 267hours, an amine antioxidant was added; and at 503 hours a mixture ofEthanox 703 and Ethanox 330 was added. Ethanox 703 is a tradedesignation for 2,6-di-tert-butyl-α-dimethylamino-p-cresol. The phenolicantioxidant added at 153 hours was a mixture of t-butyl phenolderivatives sold under the trade designation Iganox L-130 by Ciba-Geigy;and the amine antioxidant added at 267 hours was a reaction product ofN-phenylbenzylamine and 2,4,4-trimethylpentene, sold under the tradedesignation L-57 by Ciba-Geigy.

TABLE 10 TESTS Erosion Test Additives Iron Corrosion Erosion ValveEpoxide Depletion Run Basestock Phenolics Amines Inhibitor Leakage @300° F. W-1 TIBP/DIBPP Continuation Combination None <100 cc >95%^(a)  W-2 TIBP/DIBPP E703/E330 DODPA None at the start. >200 cc 65%^(a) At 22hrs. Petrolite 31001 added. W-3 TIBP/DIBPP Ionol/E702 DODPA Vanlube  100 cc 22%^(a) R1-G W-4 TIBP/DIBPP Ionol/E702 DODPA Vanlube —78.9%^(b)   R1-G W-5 TIBP/DIBPP Ionol/E702/E330 DODPA Vanlube — 58%^(a)R1-G ^(a)Boeing BMS-3-11G Erosion Resistance Test ^(b)Boeing, BMS-3-11G,Erosion Control Test

These data and those of Example 9 demonstrate that the iron corrosionresistance agents Petrolite 31001 and vanlube RI-G are both satisfactorywith respect to effect on erosion. Neither appears to significantlyaccelerate erosion, and the compositions containing these additivesexhibit satisfactory antierosion properties.

The combination of a triisobutyl phosphate/diisobutyl phenyl phosphatebase stock with the 4,5-dihydroimidazole derivative of Vanlube RI-Gprovides a remarkable and unexpectedly favorable effect on the stabilityof the composition at elevated temperature. This effect is not seen withiron corrosion inhibitors other than 4,5-dihydroimidazoles of the abovedescribed type.

EXAMPLE 12

Formulations of fluid compositions were prepared in accordance with theprocedure described in Example 1 using the quantities of materials andcomponents set forth in Table 11 to demonstrate the superiorcharacterizing properties exhibited by the fluid compositions of thepresent invention. The characterizing properties—determined inaccordance with the procedures set forth in the Boeing MaterialSpecification for Fire Resistant Hydraulic Fluid. BMS 2-11G (Rev. Jul.17, 1986) also are set forth in Table 11. In this table, “TBP” refers totributyl phosphate; “TIBP” refers to triisobutyl phosphate; “DIBPP”refers to diisobutyl phenyl phosphate; “DBPP” refers to dibutyl phenylphosphate; “DBPP (>99%)” refers to dibutyl phenyl phosphate of greaterthan 99% by weight purity; “S-154” refers to a fluid base stockcomponent comprising about 42.8% triphenyl phosphate, about 41.7%tert-butylphenyl diphenyl phosphate, about 12.8% di(tert-butylphenyl)phenyl phosphate, 1.3% tri(tert-butylphenyl) phosphate, and 1.4% lightends and other unidentified material, all such concentrations expressedby weight; “Kronitex 100” refers to tri(isopropylphenyl) phosphatecommercially available from FMC Corporation; “6703”, “6770”, “6477”, and“6961-PMN” refer to poly(alkyl methacrylate) viscosity index improverscommercially available from Rohm and Haas Company; “HF411” and “HF460”refer to poly(butyl/hexyl methacrylate) viscosity index improvers,“C₆-C₁₀ polyacrylate” refers to a viscosity index improver, commerciallyavailable from Union Carbide Corporation; “FC-98” refers to anantierosion agent comprising a potassium salt of perfluorooctylsulfonicacid, also known as perfluorooctanesulfonic acid a mixture of apotassium salt of perfluoroethyl cyclohexyl sulfonate, a potassium saltof perfluoromethyl cyclohexyl sulfonate, a potassium salt ofperfluorodimethyl cyclohexyl sulfonate, and a potassium salt ofperfluorocyclohexyl sulfonate; “NH4PF6/Ca(SO3DF3)2” refers to anantierosion agent comprising a mixture of ammonium hexafluorophosphate(NH₄PF₆) and calcium di(perfluoromethanesulfonate) [Ca(SO₃CF₃)₂]; “MCS1562” refers to 2-ethylhexyl 3,4-epoxycyclohexanecarboxylate, an acidscavenger, described in U.S. Pat. No. 3,723,320; “ERL 4234” refers to2-(3,4-epoxycyclohexyl)-5,5-spiro(3,4-epoxy)cyclohexane-m-dioxane, anacid scavenger, commercially available from Union Carbide Corporation;“DODPA” refers to di(p-octylphenyl)amine, an antioxidant; “Ionol” refersto 2,6-di-tert-butyl-p-cresol, an antioxidant, commercially availablefrom Shell Chemical Company; “E-702” refers tobis(3,5-di-tert-butyl-4-hydroxyphenyl)methane, an antioxidant,commercially available under the trade designation Ethanox® 702 fromEthyl Corporation; “E-330” refers to1,3,5-trimethyl-2,4,6-tris(3,5-di-tert-butyl hydroxyphenyl)benzene, anantioxidant, commercially available under the trade designation Ethanox®330 from Ethyl Corporation; “KOPHEN” refers to potassium phenate;“P-57068” refers to a benzotriazole derivative, a copper corrosioninhibitor, commercially available under the trade designation Petrolite57068 from Petrolite Corporation; “FH-132” refers to1,2-di(phenylthio)ethane, a copper corrosion inhibitor, “P-31001” refersto an iron corrosion inhibitor, commercially available under the tradedesignation Petrolite 31001 from Petrolite Corporation; “Vanl RI-G”refers to the condensation product of 4,5-dihydro-1H-imidazole andC₁₆-C₁₈ fatty acid commercially available under the trade designationVanlube RI-G from Vanderbilt Company; “Sarkosyl-O” refers toN-methyl-N-(1-oxo-9-octadecenyl)glycine commercially available under thetrade designation Sarkosyl®-O from Ciba-Geigy Corporation; and “UnamineC” refers to 1-hydroxyethyl-2-coca-imidazoline, an iron corrosioninhibitor.

TABLE 11¹ Skydrol ® I.D.4² Hyjet ® IVA³ 1⁴ 2⁴ 3⁵ 4⁶ 5⁷ 6⁸ PhosphateEster Base Stock TBP 50-60 72.60 67.00 67.00 TIBP 69.00 62.00 87.5582.90 DIBPP 22.00 20.00 10.00 5.00 DBPP 30-35 22.30 22.30 DBPP (>99%)S-154 Kronitex 100 11.80 Viscosity Index Improver 6703 4.65 6770 6.304.50 4.50 6477 7.50 7.50 6961-PMN HF411  5-10 HF460 C₆-C₁₀ Polyacrylate6.00 Anti-Erosion Agent FC-98, ppm 0.005-1.00  500 500 500 500 250 250NH₄PF₆/Ca(SO₄CF₃)₂ 0.078 Acid Scavenger (Epoxide) MCS 1562 4-8 6.30 6.306.30 6.30 5.80 5.80 ERL 4234 1.9-2.3 Antioxidant DODPA 1.00 0.45 0.450.45 0.45 0.45 0.45 Ionol 1.00 0.50 0.25 0.25 0.25 0.25 0.25 0.25 E-7020.90 0.90 0.90 0.45 0.45 0.45 E-330 0.45 0.45 0.45 KOPHEN, ppm 350 350350 350 Copper Corrosion Inhibitor P-57068 0.05 0.05 0.05 0.05 FH-1320.13-1.00 0.50 Iron Corrosion Inhibitor P-31001 0.05 Vant RI-G, ppm 250250 250 250 250 Sarkosyl O Unamine C 0.15 Water 0.23 0.14 0.15 0.15Char. Properties Acidity 0.09 0.02 0.1 0.1 Viscosity, cst @−65° F.(−53.8° C.) 1185 1353 1777 3957 2720 1945 1778 100° F. (37.8° C.) 11.4210.81 14.40 13.51 9.29 8.76 8.89 210° F. (98.9° C.) 3.93 3.62 5.00 4.283.09 2.95 3.03 Thermal Stability, hr @325° F. (162.8° C.) −300 — 677 4201264 1251 Bulk Modulus, psi 221,000 246,000 212,000 — 201,000 201,000211,000 Autoignition Temp., °F./°C. 880/471.1 800/426.7 870/465.6950/510 950/510 7⁹ 8¹⁰ 9¹¹ 10¹¹ 11¹¹ 12¹² 13¹³ Phosphate Ester BaseStock TBP 65.51 69.10 84.35 84.35 84.40 84.61 86.60 TIBP DIBPP 20.5418.00 DBPP 3.00 3.00 3.00 3.00 DBPP (>99%) S-154 Kronitex 100 ViscosityIndex Improver 6703 5.50 4.70 4.50 4.70 4.65 5.25 6770 6477 4.656961-PMN HF411 HF460 C₆-C₁₀ Polyacrylate Anti-Erosion Agent FC-98, ppm250 250 250 250 250 250 250 NH₄PF₆/Ca(SO₄CF₃)₂ Acid Scavenger (Epoxide)MCS 1562 6.30 6.00 6.00 5.80 5.80 5.80 6.00 ERL 4234 Antioxidant DODPA0.45 0.45 0.45 0.45 0.45 0.45 0.45 Ionol 0.25 0.25 0.25 0.25 0.25 0.250.25 E-702 0.45 0.45 0.45 0.45 0.45 0.45 0.45 E-330 0.45 0.45 0.45 0.450.45 0.45 0.45 KOPHEN, ppm Copper Corrosion Inhibitor P-57068 0.50 0.500.50 0.50 0.50 0.50 0.50 FH-132 Iron Corrosion Inhibitor P-31001 250 250250 250 250 250 250 Vant RI-G, ppm Sarkosyl O Unamine C 0.09 0.15 WaterChar. Properties Acidity 0.04 0.03 Viscosity, cst @−65° F. (−53.8° C.)2438 1822 1899 2045 1886 4226 1831 100° F. (37.8° C.) 11.15 9.46 8.869.09 9.05 12.11 9.47 210° F. (98.9° C.) 3.69 3.17 2.98 3.07 3.08 3.753.21 Thermal Stability, hr @325° F. (162.8° C.) 929 >900 1800 BulkModulus, psi 215,000 216,000 210,000 209,000 205,000 Autoignition Temp.,°F./°C. 900/482.2 900/482.2 890/476.7 880/471.1 880/471.1 ¹Except asotherwise indicated, amounts of components are expressed as % by weight.²Skydrol ® LD-4 aircraft hydraulic fluid; commercially available fromMonsanto Company. ³Hyjet ® IVA aircraft hydraulic fluid; commerciallyavailable from Chevron International Oil Company. ⁴The phosphate esterbase stock comprises 75.03% by weight TBP and 24.97% BY WEIGHT DBPP.⁵The phosphate ester base stock comprises 75.82% TIBP and 24.18% byweight DIBPP. ⁶The phosphate ester base stock comprises 73.81% TIBP and24.39% by weight DIBPP. ⁷The phosphate ester base stock comprises 89.75%TIBP and 10.25% by weight DIBPP. ⁸The phosphate ester base stockcomprises 94.31% TIBP and 5.69% by weight DIBPP. ⁹The phosphate esterbase stock comprises 76.13% TIBP and 23.87% by weight DBPP (>99%). ¹⁰Thephosphate ester base stock comprises 79.33% TIBP and 20.67% by weightDBPP (>99%). ¹¹The phosphate ester base stock comprises 96.57% TIBP, and3.43% by weight triaryl phosphate [tri-phenyl phosphate (TPP) - 42.8%,tert-butylphenyl diphenyl phosphate (TBPDP) - 41.7%,di(tert-butylphenyl)phenyl phosphate (DTBPPP) - 12.8%, andtri(tert-butylphenyl)phosphate - 1.3% (TTBPP), and 1.4% light ends andother unindentified materials]. ¹²The phosphate ester base stockcomprises 96.58% TIBP and 3.42% by weight triaryl phosphate (42.8% TPP,41.7% TBPDP, 12.8% DTBPPP, and 1.3% TTBPP). ¹³The phosphate ester basestock comprises 100.00% by weight of TIBP.

EXAMPLE 13

Tests were conducted to compare the hydrolytic stability of certainrepresentative formulations set forth in Table 11 with commerciallyavailable hydraulic fluids to demonstrate the superior hydrolyticstability of the fluid compositions of the instant invention containingeither or both the additive package suitable for use in fluidcompositions of the instant invention and the isoalkyl phosphateester-based base stocks and the isoalkyl phosphate ester/aryl phosphateester-based base stocks. In each of the tests, a 301 stainless steeltube having dimensions of approximately 1.90 cm (0.75 in) I.D.×22.86 cm(9.0 in) length and a capacity of approximately 53 cc was filled toapproximately 85% capacity (approximately 45 cc) with the fluid to betested. The head space in the tube was filled with air. The tube wascapped and heated to a predetermined test temperature—325° F. (162.7°C.)—and maintained at that temperature throughout the test. Each tubewas monitored over time and samples were taken to follow trends in thefluid's chemical composition, in particular the concentration of theacid scavenger (epoxide) present in the sample. When the epoxide is 100%depleted, the fluid is typically degraded to the point that itsusefulness as an aircraft hydraulic fluid has essentially beenexhausted. An epoxide depletion approached 100%, test samples weretitrated for acidity. When the neutralization number of the fluidreached 1.5 or greater, the test was halted and the number of hoursrecorded. The parameters and results are tabulated in Table 12.

TABLE 12 Fluid Hydrolytic Stability @ 325° F. Stability, hr. @ % WaterFluid (from Table 11) <0.2 (Neat) 0.5 Skydrol  ® LD-4¹ 300 37 Hyjet  ®IVA² 200 28  2 450 85  5 1000  310  11 >1000  350  ¹Commerciallyavailable from Monsanto Company. ²Commercially available from ChevronInternational Oil Company.

What is claimed is:
 1. A fluid composition suitable for use as anaircraft hydraulic fluid, comprising: (a) a fire resistant phosphateester base stock, the basic stock comprising between about 50% and about85% by weight of a trialkyl phosphate in which the alkyl substituentsare substantially isoalkyl C4 or C₅ and are bonded to the phosphatemoiety via a primary carbon atom, between about 18% and about 35% byweight of a dialkyl aryl phosphate in which the alkyl substituents areas previously defined, and between about 0% and about 5% by weight of analkyl diaryl phosphate in which the alkyl substituent is as previouslydefined, with the proviso that the sum of the proportionate amount ofeach base stock component must equal 100%; (b) an acid scavenger in anamount effective to neutralize phosphoric acid and phosphoric acidpartial esters formed in situ by hydrolysis of any of the phosphateesters of said base stock, wherein said acid scavenger comprises anepoxide compound; (c) an anti-erosion additive in an amount effective toinhibit flow-induced electrochemical or zeta corrosion of theflow-metering edges of hydraulic servo valves in hydraulic systems,wherein said anti-erosion additive comprises an alkali metal salt of aperfluoroalkyl sulfonic acid; (d) a viscosity index improver in anamount effective to cause the fluid composition to exhibit a viscosityof at least about 3.0 centistokes at about 210° F., at least about 9.0centistokes at about 100° F., and less than about 4200 centistokes at−65° F., wherein said viscosity index improver comprises a methacrylateester polymer at least 95% by weight of the polymer having a molecularweight of between about 50,000 and 1,500,000 ; and (e) an antioxidant inan amount effective to inhibit oxidation of fluid composition componentsin the presence of oxidizing agents, wherein said antioxidant isselected from the group consisting of at least one hindered phenol and acombination of at least one hindered phenol and at least one aminecompound.
 2. A fluid composition as set forth in claim 1 wherein saiddialkyl aryl phosphate is a dialkyl phenyl phosphate.
 3. A fluidcomposition as set forth in claim 1 wherein the viscosity index improveris present in an amount effective to cause the fluid composition toexhibit a viscosity between about 3 and about 5 centistokes at about210° F. and between about 9 and about 15 centistokes at 100° F.
 4. Afluid composition as set forth in claim 1 containing a viscosity indeximprover in a proportion of between about 3% and about 10% by weight ofthe fluid composition.
 5. A fluid composition as set forth in claim 191containing a 2,4,6-trialkylphenol in a proportion of between about0.1% and about 1.0% by weight of the fluid composition, adi(alkylphenyl)amine in a proportion of between about 0.3% and about1.0% by weight of the fluid composition, and a hindered polyphenolselected from the group consisting ofbis(3,5-dialkyl-4-hydroxyaryl)methane and1,3,5-trialkyl-2,4,6-tris(3,5-di-tert-butyl-4-hydroxyaryl)benzene andmixtures thereof, in a proportion of between about 0.3% and about 1.0%by weight of the fluid composition.
 6. A fluid composition as set forthin claim 2 wherein phosphate esters containing an aryl substituent donot constitute more than about 25% by weight of the base stock.
 7. Afluid composition suitable for use as an aircraft hydraulic fluid,comprising: (a) a fire resistant phosphate ester base stock comprisingbetween about 10% and about 90% of a trialkyl phosphate wherein thealkyl substituents are substantially isoalkyl C₄ and C₅ and are bondedto the phosphate moiety via a primary carbon atom, between about 0% andabout 70% by weight of a dialkyl aryl phosphate wherein the alkylsubstituents are as previously defined, and between about 0% and about25% by weight of an alkyl diaryl phosphate wherein the alkyl substituentis as previously defined, with the proviso that the sum of theproportionate amount of each base stock component must equal 100%; (b) aviscosity index improver in a proportion of between about 3% and about10% by weight of the fluid composition, the viscosity index improvercomprising a methacrylate ester polymer, the repeating units of whichsubstantially comprise butyl and hexyl methacrylate, at least 95% byweight of the polymer having a molecular weight of between about 50,000and about 1,500,000; (c) an anti-erosion agent in a proportion ofbetween about 0.02% and about 0.08% by weight of the fluid composition,the anti-erosion agent comprising an alkali metal salt of aperfluoroalkyl sulfonic acid, the alkyl substituent of which is selectedfrom the group consisting of hexyl, heptyl, octyl, nonyl, decyl, andmixtures thereof; (d) an acid scavenger in a proportion of between about1.5% and about 10% by weight of the fluid composition, the acidscavenger comprising an epoxide compound; (e) a 2,4,6-trialkylphenol ina proportion of between about 0.1% and about 1.0% by weight of the fluidcomposition; (f) a di(alkylphenyl)amine in a proportion of between about0.3% and about 1.0% by weight of the fluid composition; and (g) ahindered polyphenol selected from the group consisting ofbis(3,5-dialkyl-4-hydroxyaryl)methane,1,3,5-trimethyl-2,4,6-tris(3,5-di-tert-butyl)-4 -hydroxyaryl)benzene andmixtures thereof in a proportion of between about 0.3% and about 1.0% byweight of the fluid composition.
 8. A fluid composition suitable for useas an aircraft hydraulic fluid, comprising: (a) a fire resistantphosphate ester base stock comprising between about 10% and about 90% byweight of a trialkyl phosphate wherein the alkyl substituents aresubstantially isoalkyl C₄ or C₅ and are bonded to the phosphate moietyvia a primary carbon atom, between about 0% and about 70% by weight of adialkyl aryl phosphate wherein the alkyl substituents are as previouslydefined, and between about 0% and about 25% by weight of an alkyl diarylphosphate wherein the alkyl substituent is as previously defined, withthe proviso that the sum of the proportionate amount of each base stockcomponent must equal 100%; (b) a viscosity index improver in aproportion of between about 3% and about 10% by weight of the fluidcomposition, the viscosity index improver comprising a methacrylateester polymer, the repeating units of which substantially comprise butyland hexyl methacrylate, at least 95% by weight of the polymer having amolecular weight of between about 50,000 and about 1,500,000; (c) ananti-erosion agent in a proportion of between about 0.02% and about0.08% by weight of the fluid composition, the anti-erosion agentcomprising an alkali metal salt of a perfluoroalkylsulfonic acid, thealkyl substituent of which is selected from the group consisting ofhexyl, heptyl, octyl, nonyl, decyl, and mixtures thereof; (d) an acidscavenger in a proportion of between about 1.5% and about 10% by weightof the composition, the acid scavenger selected from the groupconsisting of a derivative of a 3,4-epoxy cyclohexane carboxylate and adiepoxide compound corresponding to the formula

wherein R³ is an organic group containing 1 to 10 carbon atoms, from 0to 6 oxygen atoms and from 0 to 6 nitrogen atoms, and R⁴ through R⁹ areindependently selected from among hydrogen and aliphatic groupscontaining 1 to 5 carbon atoms, and mixtures of the 3,4-epoxycyclohexanecarboxylate and the diepoxide compound; (e) 2,4,6-trialkylphenol in aproportion of between about 0.1% and about 1.0% by weight of thecomposition; (f) a di(alkylphenyl)amine in a proportion of between about0.3% and about 1.0% by weight of the fluid composition; and (g) ahindered polyphenol selected from the group consisting ofbis(3,5-dialkyl-4-hydroxyaryl)methane,1,3,5-trimethyl-2,4,6-tris(3,5-di-tert-butyl-4 -hydroxyaryl)benzene, andmixtures thereof in a proportion of between about 0.3% and about 1.0% byweight of the fluid composition.
 9. A fluid composition as set forth inclaim 8 further comprising benzotriazole or a benzotriazole derivativein a proportion of between about 0,005 0.005% and about 0.09% as acopper corrosion inhibitor.
 10. A fluid composition as set forth inclaim 8 further comprising between about 0.0035% and about 0.010% byweight of an alkali metal arenate.
 11. A fluid composition suitable foruse as an aircraft hydraulic fluid, comprising: (a) a fire resistantphosphate ester base stock comprising between about 35% and about 90% byweight of a trialkyl phosphate wherein the alkyl substituents aresubstantially isoalkyl C₄ and C₅ and are bonded to the phosphate moietyvia a primary carbon atom, between about 0% and about 35% by weight of adialkyl aryl phosphate wherein the alkyl substituents are as previouslydefined, and between about 0% and about 20% by weight of a triarylphosphate, with the proviso that the sum of the proportionate amount ofeach base stock component must equal 100%; (b) a viscosity indeximprover in a proportion of between about 3% and about 10% by weight ofthe fluid composition, the viscosity index improver comprising amethacrylate ester polymer, the repeating units of which substantiallycomprise butyl and hexyl methacrylate, at least 95% by weight of thepolymer having a molecular weight of between about 50,000 and about1,500,000; (c) an anti-erosion agent in a proportion of between about0.02% and about 0.08% by weight of the fluid composition, theanti-erosion agent comprising an alkali metal salt of a perfluoroalkylsulfonic acid, the alkyl substituent of which is selected from the groupconsisting of hexyl, heptyl, octyl, nonyl, decyl, and mixtures thereof;(d) an acid scavenger in a proportion of between about 1.5% and about10% by weight of the fluid composition, the acid scavenger selected fromthe group consisting of a derivative of a 3,4-epoxy cyclohexanecarboxylate and a diepoxide compound corresponding to the formula

wherein R³ is an organic group containing 1 to 10 carbon atoms, from 0to 6 oxygen atoms, and from 0 to 6 nitrogen atoms, and R⁴ through R⁹ areindependently selected from among hydrogen and aliphatic groupscontaining 1 to 5 carbon atoms, and mixtures of the3,4-epoxycyclohexanecarboxylate and the diepoxide compound; (e) a2,4,6-trialkylphenol in a proportion or between about 0.1% and about1.0% by weight of the fluid composition; (f) a di(alkylphenyl)amine in aproportion of between about 0.3% and about 1.0% by weight of the fluidcomposition; and (g) a hindered polyphenol selected from the groupconsisting of bis(3,5-dialkyl-4-hydroxyaryl) methane,1,3,5-trimethyl-2,4,6-tris(3,5-di-tert-butyl-4-hydroxyaryl)benzene andmixtures thereof in a proportion of between about 0.3% and about 1.0% byweight of the fluid composition.
 12. A fluid composition as set forth inclaim 11 wherein the base stock comprises between about 80% and about90% by weight of the trialkyl phosphate and between about 10% and about208 by weight of a tri(alkylaryl)phosphate.
 13. A fluid composition asset forth in claim 12 wherein the tri(alkylaryl) phosphate is selectedfrom the group consisting of tri(isopropylphenyl)phosphate,tri(isobutylphenyl)phosphate, and tri(tert-butylphenyl)phosphate.
 14. Afluid composition as set forth in claim 8 wherein the base stockcomprises between about 10% and about 72% by weight of the trialkylphosphate, between about 18% and about 70% by weight of the dialkyl arylphosphate, and between about 0% and about 25% by weight of the alkyldiaryl phosphate.
 15. A fluid composition as set forth in claim 14wherein the base stock comprises between about 10% and about 25% byweight of the trialkyl phosphate, between about 45% and about 70% byweight of the dialkyl aryl phosphate, and between about 5% and about 25%by weight of the alkyl diaryl phosphate.
 16. A fluid composition as setforth in claim 14 wherein the base stock comprises between about 50% andabout 72% by weight of the trialkyl phosphate, between about 18% andabout 35% by weight of the dialkyl aryl phosphate, and between about 0%and about 10% by weight of the alkyl diaryl phosphate.
 17. A fluidcomposition as set forth in claim 8 wherein the base stock containsbetween about 0% and about 5% by weight of the alkyl diaryl phosphate.18. A composition as set forth in claim 8 wherein said alkylsubstituents are substantially isobutyl or isopentyl.
 19. A fluidcomposition suitable for use as an aircraft hydraulic fluid, comprising:(a) a fire resistant phosphate ester base stock, the base stockcomprising between about 10 and about 90% by weight of a trialkylphosphate wherein the alkyl substituents are substantially isobutyl orisopentyl and are bonded to the phosphate moiety via a primary carbonatom, between about 0% and about 70% by weight of a dialkyl arylphosphate wherein the alkyl substituents are as previously defined, andbetween about 0% and about 25% by weight of an alkyl diaryl phosphatewherein the alkyl substituent is as previously defined, with the provisothat the sum of the proportionate amount of each base stock componentmust equal 100%; (b) an acid scavenger in an amount effective toneutralize phosphoric acid partial esters formed in situ by hydrolysisof any of the phosphate esters of the base stock, wherein said acidscavenger comprises an epoxide compound; (c) an anti-erosion additive inan amount effective to inhibit flow-induced electrochemical or zetacorrosion of the flow-metering edges of hydraulic servo valves inhydraulic systems, wherein said anti-erosion additive comprises analkali metal salt of a perfluoroalkyl sulfonic acid; (d) a viscosityindex improver in an amount effective to cause the fluid composition toexhibit a viscosity of at least about 3.0 centistokes at about 210° F.,at least about 9.0 centistokes at about 100° F., and less than about4200 centistokes at −65° F., wherein said viscosity index improvercomprises a methacrylate ester polymer at least 95% by weight of thepolymer having a molecular weight of between about 50,000 and 1,500,000; and (e) an antioxidant in an amount effective to inhibit oxidation offluid composition components in the presence of oxidizing agents,wherein said antioxidant is selected from the group consisting of atleast one hindered phenol and a combination of at least one hinderedphenol and at least one amine compound.
 20. A fluid composition as setforth in claim 19 wherein the trialkyl phosphate is triisobutylphosphate and the dialkyl aryl phosphate is diisobutyl phenyl phosphate.21. A fluid composition as set forth in claim 19 further comprisingbenzotriazole or a benzotriazole derivative in a proportion of betweenabout 0.005% and about 0.09% by weight as a copper corrosion inhibitor.22. A fluid composition as set forth in claim 19 further comprising a4,5-dihydroimidazole compound corresponding to the formula

where R¹ is selected from the group consisting of hydrogen, alkyl,alkenyl, hydroxyalkyl, hydroxyalkenyl, alkoxyalkyl and alkoxyalkenyl,and R² is selected from the group consisting of alkyl, alkenyl andaliphatic carboxylate.
 23. A fluid composition as set forth in claim 22wherein the 4,5-dihydroimidazole is selected from the group consistingof 2-(8-heptadecenyl)-4,5-dihydro-1H-imidazole-1-ethanol and thecondensation product of a C₁₄ to C₁₈ fatty acid and4,5-dihydro-1H-imidazole.
 24. A fluid composition as set forth in claim23 wherein the 4,5-dihydroimidazole compound is the condensation productof a C₁₆ to C₁₈ fatty acid and 4,5-dihydro-1H-imidazole.
 25. A fluidcomposition as set forth in claim 22 19wherein the hindered phenolantioxidant comprises a mixture of a hindered phenol and a hinderedpolyphenol.
 26. A fluid composition as set forth in claim 25 wherein thehindered phenol polyphenol comprises a hindered polyphenol compoundselected from the group consisting of bis(3,5-dialkyl-4-hydroxyaryl)methane, 1,3,5-trialkyl-2,4,6-tris(3,5-di-tert-butyl-4-hydroxyaryl) benzene, and mixtures thereof.
 27. A fluidcomposition as set forth in claim 25 wherein the antioxidant furthercomprises an amine compound.
 28. A fluid composition as set forth inclaim 27 25wherein the antioxidant amine compound is a diarylamine. 29.A fluid composition as set forth in claim 28 wherein the diarylamine isdi(p-octylphenyl)amine.
 30. A fluid composition as set forth in claim 28further comprising up to about 0.7% by weight of2,6-di-tert-butyl-p-cresol.
 31. A fluid composition suitable for use asan aircraft hydraulic fluid, comprising: (a) a fire resistant phosphateester base stock, the base stock comprising between about 10% and about90% by weight of a trialkyl phosphate wherein the alkyl substituents aresubstantially isoalkyl C₄ or C₅ and are bonded to the phosphate moietyvia a primary carbon atom, between about 0% and about 70% by weight of adialkyl aryl phosphate wherein the alkyl substituents are as previouslydefined, and between about 0% and about 25% by weight of an alkyl diarylphosphate wherein the alkyl substituent is as previously defined; (b) anacid scavenger in an amount effective to neutralize phosphoric acid andphosphoric acid partial esters formed in situ by hydrolysis of any ofthe phosphate esters of the base stock, wherein said acid scavengercomprises an epoxide compound; (c) an anti-erosion additive in an amounteffective to inhibit flow-induced electrochemical or zeta corrosion ofthe flow-metering edges of hydraulic servo valves in hydraulic systems,wherein said anti-erosion additive comprises an alkali metal salt of aperfluoroalkyl sulfonic acid; (d) a viscosity index improver in anamount effective to cause the fluid composition to exhibit a viscosityof at least about 3.0 centistokes at about 210° F., at least about 9.0centistokes at about 100° F., and less than about 4200 centistokes at−65° F., wherein said viscosity index improver comprises a methacrylateester polymer at least 95% by weight of the polymer having a molecularweight of between about 50,000 and 1,500,000 ; (e) an antioxidant in anamount effective to inhibit oxidation of fluid composition components inthe presence of oxidizing agents, wherein said antioxidant is selectedfrom the group consisting of at least one hindered phenol and acombination of at least one hindered phenol and at least one aminecompound; and (f) a 4,5-dihydroimidazole compound in an amount effectiveto increase the stability of the composition by at least 25% at 300° F.as measured by epoxide depletion, the 4,5-dihydroimidazole compoundcorresponding to the formula

wherein R¹ is selected from the group consisting of hydrogen, alkyl,alkenyl, hydroxyalkyl, hydroxyalkenyl, alkoxyalkyl and alkoxyalkenyl,and R² is selected from the group consisting of alkyl, alkenyl andaliphatic carboxylate.
 32. A fluid composition as set forth in claim 31wherein R¹ is hydrogen or lower alkyl and R² is a fatty acid residue.33. A fluid composition as set forth in claim 31 wherein R¹ ishydroxyalkyl and R² is alkenyl.
 34. A fluid composition as set forth inclaim 31 wherein the 4,5-dihydroimidazole is selected from the groupconsisting of 2-(8-heptadecenyl)-4,5-dihydro-1H-imidazole-1-ethanol andthe condensation product of a C₁₄ to C₁₈ fatty acid and4,5-dihydro-1H-imidazole.
 35. A fluid composition as set forth in claim34 wherein the 4,5-dihydroimidazole compound is the condensation productof a C₁₆ to C₁₈ fatty acid and 4,5-dihydro-1H-imidazole.
 36. Acomposition as set forth in claim 35 wherein said alkyl substituents ofsaid trialkyl phosphate and said dialkyl aryl phosphate aresubstantially isobutyl or isopentyl.
 37. A fluid composition as setforth in claim 31 wherein the hindered phenol antioxidant comprises amixture of a hindered phenol and a hindered polyphenol.
 38. A fluidcomposition as set forth in claim 37 wherein the hindered phenolpolyphenol comprises a hindered polyphenol compound selected from thegroup consisting of bis(3,5-dialkyl-4-hydroxyaryl)methane,1,3,5-trialkyl-2,4,6-tris(3,5-di-tert-butyl-4-hydroxyaryl)benzene, andmixtures thereof.
 39. A fluid composition as set forth in claim 37wherein the antioxidant further comprises an amine compound.
 40. A fluidcomposition as set forth in claim 39 31further comprisingwherein theantioxidant amine compound is a diarylamine antioxidant .
 41. Acomposition as set forth in claim 40 wherein said diarylamine comprisesdi(p-octylphenyl)amine.
 42. A composition as set forth in claim 40further comprising up to about 1.0% by weight of 2,6-di-tertiary-butylp-cresol.
 43. A composition as set forth in claim 31 wherein said alkylsubstituents of said trialkyl phosphate and said dialkyl aryl phosphateare substantially isobutyl or isopentyl.
 44. A fluid composition as setforth in claim 1 wherein the viscosity index improver is a methacrylateester polymer, the repeating units of which substantially comprise butyland hexyl methacrylate, at least 95% by weight of the polymer having amolecular weight of between about 50,000 and about 1,500,000.
 45. Afluid composition as set forth in claim 19 further comprising ananti-foam agent.
 46. A fluid composition as set forth in claim 45wherein the anti-foam agent is a polyalkylsiloxane.
 47. A fluidcomposition as set forth in claim 46 wherein the polyalkylsiloxane ispolymethylsiloxane.
 48. A fluid composition as set forth in claim 45wherein the anti-foam agent is present in a concentration of from about0.0001% to about 0.001% by weight of the fluid composition.
 49. A fluidcomposition as set forth in claim 48 wherein the anti-foam agent ispresent in a concentration of about 0.0005% by weight of the fluidcomposition.
 50. A fluid composition suitable for use as an aircrafthydraulic fluid, comprising: (a) a fire resistant phosphate ester basestock, the base stock comprising between about 10% and about 100% byweight of a trialkyl phosphate in which the alkyl substituents aresubstantially isoalkyl C₄ or C₅ and are bonded to the phosphate moietyvia a primary carbon atom, between about 0% and about 70% by weight of adialkyl aryl phosphate in which the alkyl substituents are as previouslydefined, and between about 0% and about 25% by weight of an alkyl diarylphosphate in which the alkyl substituent is as previously defined, withthe proviso that the sum of the proportionate amount of each base stockcomponent must equal 100%; (b) an acid scavenger in an amount effectiveto neutralize phosphoric acid partial esters formed in situ byhydrolysis of any of the phosphate esters of the base stock, whereinsaid acid scavenger comprises an epoxide compound; (c) an anti-erosionadditive in an amount effective to inhibit flow-induced electrochemicalor zeta corrosion of the flow-metering edges of hydraulic servo valvesin hydraulic systems, wherein said anti-erosion additive comprises analkali metal salt of a perfluoroalkyl sulfonic acid; (d) a viscosityindex improver in an amount effective to cause the fluid composition toexhibit a viscosity of at least about 3.0 centistokes at about 210° F.,at least about 9.0 centistokes at about 100° F., and less than about4200 centistokes at −65° F., wherein said viscosity index improvercomprises a methacrylate ester polymer at least 95% by weight of thepolymer having a molecular weight of between about 50,000 and 1,500,000; and (e) an antioxidant in an amount effective to inhibit oxidation offluid composition components in the presence of oxidizing agents,wherein said antioxidant is selected from the group consisting of atleast one hindered phenol and a combination of at least one hinderedphenol and at least one amine compound.
 51. A fluid composition as setforth in claim 50 further comprising benzotriazole or a benzotriazolederivative in a proportion of between about 0.005% and about 0.09% byweight as a copper corrosion inhibitor.
 52. A fluid composition as setforth in claim 50 or 51 further comprising a 4,5-dihydroimidazolecompound corresponding to the formula

wherein R¹ is selected from the group consisting of hydrogen, alkyl,alkenyl, hydroxyalkyl, hydroxyalkenyl, alkoxyalkyl and alkoxyalkenyl,and R² is selected from the group consisting of alkyl, alkenyl andaliphatic carboxylate.
 53. A fluid composition as set forth in claim 52wherein the 4,5-dihydroimidazole compound is present in a proportion ofbetween about 0.01% and about 0.1% by weight of the fluid composition.54. A fluid composition as set forth in claim 52 wherein the4,5-dihydroimidazole is selected from the group consisting of2-(8-heptadecenyl)-4,5-dihydro-1H-imidazole-1-ethanol and thecondensation product of a C₁₄ to C₁₅ fatty acid and4,5-dihydro-1H-imidazole.
 55. A fluid composition as set forth in claim54 wherein the 4,5-dihydroimidazole compound is the condensation productof a C16 to C₁₈ fatty acid and 4,5-dihydro-1H-imidazole.
 56. A fluidcomposition suitable for use as an aircraft hydraulic fluid, comprising:(a) a fire resistant phosphate ester base stock, said base stockcomprising between about 10% and about 100% by weight of a trialkylphosphate in which the alkyl substituents are substantially isoalkyl C₄or C₅ and are bonded to the phosphate moiety via a primary carbon atom,between about 0% and about 70% by weight of a dialkyl aryl phosphate inwhich the alkyl substituents are as previously defined, and betweenabout 0% and about 25% by weight of an alkyl diaryl phosphate in whichthe alkyl substituent is as previously defined, with the proviso thatthe sum of the proportionate amount of each base stock component mustequal 100%; (b) an acid scavenger in an amount effective to neutralizephosphoric acid and phosphoric acid partial esters formed in situ byhydrolysis of any of the phosphate esters of the base stock, whereinsaid acid scavenger comprises an epoxide compound; (c) an anti-erosionadditive in an amount effective to inhibit flow-induced electrochemicalor zeta corrosion of the flow-metering edges of hydraulic servo valvesin hydraulic systems, wherein said anti-erosion additive comprises analkali metal salt of a perfluoroalkyl sulfonic acid; (d) a viscosityindex improver in an amount effective to cause the fluid composition toexhibit a viscosity of at least about 3.0 centistokes at about 210° F.,at least about 9.0 centistokes at about 100° F., and less than about4200 centistokes at −65° F., wherein said viscosity index improvercomprises a methacrylate ester polymer at least 95% by weight of thepolymer having a molecular weight of between about 50,000 and 1,500,000; and (e) an antioxidant in an amount effective to inhibit oxidation offluid composition components in the presence of oxidizing agents,wherein said antioxidant is selected from the group consisting of atleast one hindered phenol and a combination of at least one hinderedphenol and at least one amine compound.
 57. A fluid composition as setforth in claim 56 wherein the phosphate esters containing an arylsubstituent do not constitute more than about 25% by weight of the basestock.
 58. A fluid composition as set forth in claim 56 wherein theviscosity index improver is present in a proportion of between about 3%and about 10% by weight of the fluid composition.
 59. A fluidcomposition as set forth in claim 56wherein the viscosity index improveris a methacrylate ester polymer, the repeating units of whichsubstantially comprise butyl and hexyl methacrylate, at least 95% byweight of the polymer having a molecular weight of between about 50,000and about 1,500,000wherein the hindered phenol antioxidant comprises amixture of a hindered phenol and a hindered polyphenol.
 60. A fluidcomposition as set forth in claim 56 98wherein the antioxidant comprisesa 2,4,6-trialkylphenol in a proportion of between about 0.1% and about1.0% by weight of the composition, a di(alkylphenyl)amine in aproportion of between about 0.3% and about 1.0% by weight of the fluidcomposition, and a hindered polyphenol selected from the groupconsisting of bis(3,5-dialkyl-4-hydroxyaryl)methane and1,3,5-trialkyl-2,4,6-tris(3,5-di-tert-butyl-4-hydroxyaryl)benzene, andmixtures thereof, in a proportion of between about 0.3% and about 1.0%by weight of the fluid composition.
 61. A fluid composition as set forthin claim 60 wherein the 2,4,6-trialkylphenol is2,6-di-tert-butyl-p-cresol and the hindered polyphenol is selected fromthe group consisting of bis (3,5-di-tert-butyl-4-hydroxyphenyl)methane,1,3,5-trimethyl-2,4,6-tris (3,5-di-tert-butyl-4 -hydroxyphenyl)benzene,and mixtures thereof.
 62. A fluid composition as set forth in claim 56further comprising benzotriazole or a benzotriazole derivative in aproportion of between about 0.005% and about 0.09% by weight as a coppercorrosion inhibitor.
 63. A fluid composition as set forth in claim 56 or62 further comprising a 4,5-dihydroimidazole compound corresponding tothe formula

where R¹ is selected from the group consisting of hydrogen, alkyl,alkenyl, hydroxyalkyl, hydroxyalkenyl, alkoxyalkyl and alkoxyalkenyl,and R² is selected from the group consisting of alkyl, alkenyl andaliphatic carboxylate.
 64. A fluid composition as set forth in claim 63wherein the 4,5-dihydroimidazole compound is present in a proportion ofbetween about 0.01% and about 0.1% by weight of the fluid composition.65. A fluid composition as set forth in claim 63 wherein the4,5-dihydroimidazole is selected from the group consisting of2-(8-heptadecenyl)-4,5 -dihydro-1H-imidazole-1-ethanol and thecondensation product of a C₁₄ to C₁₈ fatty acid and4,5-dihydro-1H-imidazole.
 66. A fluid composition as set forth in claim65 wherein the 4,5-dihydroimidazole compound is the condensation productof a C₆ to C₈ fatty acid and 4,5-dihydro-1H-imidazole.
 67. A fluidcomposition suitable for use as an aircraft hydraulic fluid, comprising:(a) a fire resistant phosphate ester base stock, the base stockcomprising between about 10% and about 100% by weight of a trialkylphosphate in which the alkyl substituents are substantially isoalkyl C₄or C₅ and are bonded to the phosphate moiety via a primary carbon atom,between about 0% and about 35% by weight of a dialkyl aryl phosphate inwhich the alkyl substituents are as previously defined, and betweenabout 0% and about 20% by weight of a triaryl phosphate, with theproviso that the sum of the proportionate amount of each base stockcomponent must equal 100%; (b) an acid scavenger in an amount effectiveto neutralize phosphoric acid partial esters formed in situ byhydrolysis of any of the phosphate esters of the base stock, whereinsaid acid scavenger comprises an epoxide compound; (c) an anti-erosionadditive in an amount effective to inhibit flow-induced electrochemicalor zeta corrosion of the flow-metering edges of hydraulic servo valvesin hydraulic systems, wherein said anti-erosion additive comprises analkali metal salt of a perfluoroalkyl sulfonic acid; (d) a viscosityindex improver in an amount effective to cause the fluid composition toexhibit a viscosity of at least about 3.0 centistokes at about 210° F.,at least about 9.0 centistokes at about 100° F., and less than about4200 centistokes at −65° F., wherein said viscosity index improvercomprises a methacrylate ester polymer at least 95% by weight of thepolymer having a molecular weight of between about 50,000 and 1,500,000; and (e) an antioxidant in an amount effective to inhibit oxidation offluid composition components in the presence of oxidizing agents,wherein said antioxidant is selected from the group consisting of atleast one hindered phenol and a combination of at least one hinderedphenol and at least one amine compound.
 68. A fluid composition as setforth in claim 67 wherein the phosphate esters containing an arylsubstituent do not constitute more than about 25% by weight of the basestock.
 69. A fluid composition as set forth in claim 67 wherein theviscosity index improver is present in a proportion of between about 3%and about 10% by weight of the fluid composition.
 70. A fluidcomposition as set forth in claim 67wherein the viscosity index improveris a methacrylate ester polymer, the repeating units of whichsubstantially comprise butyl and hexyl methacrylate, at least 95% byweight of the polymer having a molecular weight of between about 50,000and about 1,500,000wherein the hindered phenol antioxidant comprises amixture of a hindered phenol and a hindered polyphenol.
 71. A fluidcomposition as set forth in claim 67 99wherein the antioxidant comprisesa 2,4,6-trialkylphenol in a proportion of between about 0.1% and about1.0% by weight of the composition, a di(alkylphenyl)amine in aproportion of between about 0.3% and about 1.0% by weight of the fluidcomposition, and a hindered polyphenol selected from the groupconsisting of bis(3,5-dialkyl-4-hydroxyaryl)methane and1,3,5-trialkyl-2,4,6-tris(3,5-di-tert-butyl-4-hydroxyaryl)benzene, andmixtures thereof, in a proportion of between about 0.3% and about 1.0%by weight of the fluid composition.
 72. A fluid composition as set forthin claim 71 wherein the 2,4,6-trialkylphenol is2,6-di-tert-butyl-p-cresol and the hindered polyphenol is selected fromthe group consisting of bis(3,5-di-tert-butyl-4-hydroxyphenyl)methane,1,3,5-trimethyl-2,4,6-tris(3,5-di-tert-butyl-4-hydroxyphenyl)benzene,and mixtures thereof.
 73. A fluid composition as set forth in claim 67further comprising benzotriazole or a benzotriazole derivative in aproportion of between about 0.005% and about 0.09% by weight as a coppercorrosion inhibitor.
 74. A fluid composition as set forth in claim 67 or73 further comprising a 4,5-dihydroimidazole compound corresponding tothe formula

where R¹ is selected from the group consisting of hydrogen, alkyl,alkenyl, hydroxyalkyl, hydroxyalkenyl, alkoxyalkyl and alkoxyalkenyl,and R² is selected from the group consisting of alkyl, alkenyl andaliphatic carboxylate.
 75. A fluid composition as set forth in claim 74wherein the 4,5-dihydroimidazole compound is present in a proportion ofbetween about 0.01% and about 0.1% by weight of the fluid composition.76. A fluid composition as set forth in claim 74 wherein the4,5-dihydroimidazole is selected from the group consisting of2-(8-heptadecenyl)-4,5-dihydro-1H-imidazole-1-ethanol and thecondensation product of a C₁₄ to C₁₅ fatty acid and4,5-dihydro-1H-imidazole.
 77. A fluid composition as set forth in claim16 wherein the 4,5-dihydroimidazole compound is the condensation productof a C₁₆ to C₁₈ fatty acid and 4,5-dihydro-1H-imidazole.
 78. A fluidcomposition suitable for use as an aircraft hydraulic fluid, comprising:(a) a fire resistant phosphate ester base stock comprising between about35% and about 99% by weight of a trialkyl phosphate wherein the alkylsubstituents are substantially isoalkyl C4 or C₅ and are bonded to thephosphate moiety via a primary carbon atom, between about 0% and about35% by weight of a dialkyl aryl phosphate wherein the alkyl substituentsare as previously defined, and between about 0% and about 20% by weightof a triaryl phosphate, with the proviso that the sum of theproportionate amount of each base stock component must equal 100%; (b) aviscosity index improver in a proportion of between about 3% and about10% by weight of the fluid composition, the viscosity index improvercomprising a methacrylate ester polymer, the repeating units of whichsubstantially comprise butyl and hexyl methacrylate, at least 95% byweight of the polymer having a molecular weight of between about 50,000and about 1,500,000; (c) an anti-erosion agent in a proportion ofbetween about 0.02% and about 0.08% by weight of the fluid composition,the anti-erosion agent comprising an alkali metal salt of aperfluoroalkyl sulfonic acid, the alkyl substituent of which is selectedfrom the group consisting of hexyl, heptyl, octyl, nonyl, decyl, andmixtures there of; (d) an acid scavenger in a proportion of betweenabout 1.5% and about 10% by weight of the fluid composition, the acidscavenger selected from the group consisting of a derivative of a3,4-epoxy cyclohexane carboxylate and a diepoxide compound correspondingto the formula

wherein R³ is an organic group containing 1 to 10 carbon atoms, from 0to 6 oxygen atoms and from 0 to 6 nitrogen atoms, and R⁴ through R⁹ areindependently selected from among hydrogen and aliphatic groupscontaining 1 to 5 carbon atoms, and mixtures of the 3,4-epoxycyclohexanecarboxylate and the diepoxide compound; (e) 2,4,6-trialkylphenol in aproportion of between about 0.1% and about 1.0% by weight of the fluidcomposition; (f) a di(alkylphenyl)amine in a proportion of between about0.3% and about 1.0% by weight of the fluid composition; and (g) ahindered polyphenol selected from the group consisting ofbis(3,5-dialkyl-4-hydroxyaryl)methane,1,3,5-trimethyl-2,4,6-tris(3,5-di-tert-butyl-4-hydroxyaryl)benzene andmixtures thereof in a proportion of between about 0.3% and about 1.0% byweight of the fluid composition.
 79. A fluid composition as set forth inclaim 78 wherein the base stock comprises between about 80% and about99% by weight of the trialkyl phosphate and between about 1.0% and about20% by weight of the triaryl phosphate wherein the aryl substituents arealkyl-substituted phenyl.
 80. A fluid composition as set forth in claim79 wherein the alkyl substituent of the alkyl-substituted phenyl is C₂to C₉.
 81. A fluid composition as set forth in claim 80 wherein thealkyl substituent of the alkyl-substituted phenyl is C₃ to C₄.
 82. Afluid composition as set forth in claim 81 wherein the alkyl-substitutedphenyl is selected from the group consisting of isopropylphenyl,isobutylphenyl, and tert-butylphenyl.
 83. A fluid composition as setforth in claim 79 wherein the triaryl phosphate is a tri(alkylaryl)phosphate selected from the group consisting oftri(isopropylphenyl)phosphate, tri(isobutylphenyl) phosphate, andtri(tert-butylphenyl) phosphate.
 84. A fluid composition as set forth inclaim 83 wherein the tri(alkylaryl) phosphate is tri(tert-butylphenyl)phosphate.
 85. A fluid composition as set forth in claim 78 furthercomprising benzotriazole or a benzotriazole derivative in a proportionof between about 0.005% and about 0.09% by weight as a copper corrosioninhibitor.
 86. A fluid composition as set forth in claim 78 or 85further comprising a 4,5-dihydroimidazole compound corresponding to theformula

where R¹ is selected from the group consisting of hydrogen, alkylalkenyl, hydroxyalkyl, hydroxyalkenyl, alkoxyalkyl and alkoxyalkenyl,and R² is selected from the group consisting of alkyl, alkenyl andaliphatic carboxylate.
 87. A fluid composition as set forth in claim 86wherein the 4,5-dihydroimidazole compound is present in a proportion ofbetween about 0.01% and about 0.1% by weight of the fluid composition.88. A fluid composition as set forth in claim 86 wherein the4,5-dihydroimidazole is selected from the group consisting of2-(8-heptadecenyl)-4,5-dihydro-1H-imidazole-1-ethanol and thecondensation product of a C₁₄ to C₁₈ fatty acid and4,5-dihydro-1H-imidazole.
 89. A fluid composition as set forth in claim88 wherein the 4,5-dihydroimidazole compound is the condensation productof a C₁₆ to C₁₈ fatty acid and 4,5-dihydro-1H-imidazole.
 90. A fluidcomposition as set forth in claim 1 wherein the hindered phenolantioxidant comprises a mixture of a hindered phenol and a hinderedpolyphenol.
 91. A fluid composition as set forth in claim 90 wherein theamine antioxidant is a diaryl amine.
 92. A fluid composition as a setforth in claim 1 wherein said epoxide acid scavenger is selected fromthe group consisting of a derivative of a 3,4-epoxy cyclohexanecarboxylate and a diepoxide compound corresponding to the formula

wherein R ³ is an organic group containing 1 to 10 carbon atoms, from 0to 6 oxygen atoms and from 0 to 6 nitrogen atoms, and R ⁴ through R ⁹are independently selected from among hydrogen and aliphatic groupscontaining 1 to 5 carbon atoms, and mixtures of the 3,4-epoxycyclohexane carboxylate and the diepoxide compound.
 93. A fluidcomposition as set forth in claim 7 wherein said epoxide acid scavengeris selected from the group consisting of a derivative of a 3,4-epoxycyclohexane carboxylate and a diepoxide compound corresponding to theformula

wherein R ³ is an organic group containing 1 to 10 carbon atoms, from 0to 6 oxygen atoms and from 0 to 6 nitrogen atoms, and R ⁴ through R ⁹are independently selected from among hydrogen and aliphatic groupscontaining 1 to 5 carbon atoms, and mixtures of the 3,4-epoxycyclohexane carboxylate and the diepoxide compound.
 94. A fluidcomposition as set forth in claim 50 wherein the hindered phenolantioxidant comprises a mixture of a hindered phenol and a hinderedpolyphenol.
 95. A fluid composition as set forth in claim 94 wherein theamine antioxidant is a diaryl amine.
 96. A fluid composition as setforth in claim 95 containing a 2,4,6-trialkylphenol in a proportion ofbetween about 0.1 and about 1.0 % by weight of the fluid composition, adi(alkylphenyl)amine in a proportion of between about 0.3 % and about1.0 % by weight of the fluid composition, and a hindered polyphenolselected from the group consisting of bis( 3,5 -dialkyl- 4-hydroxyaryl)methane and 1,3,5 -trialkyl- 2,4,6 -tris( 3,5-di-tert-butyl- 4 -hydroxyaryl)benzene and mixtures thereof, in aproportion of between about 0.3 % and about 1.0 % by weight of the fluidcomposition.
 97. A fluid composition as a set forth in claim 50 whereinsaid epoxide acid scavenger is selected from the group consisting of aderivative of a 3,4-epoxy cyclohexane carboxylate and a diepoxidecompound corresponding to the formula

wherein R ³ is an organic group containing 1 to 10 carbon atoms, from 0to 6 oxygen atoms and from 0 to 6 nitrogen atoms, and R ⁴ through R ⁹are independently selected from among hydrogen and aliphatic groupscontaining 1 to 5 carbon atoms, and mixtures of the 3,4-epoxycyclohexane carboxylate and the diepoxide compound.
 98. A fluidcomposition as set forth in claim 59 wherein the amine antioxidant is adiaryl amine.
 99. A fluid composition as set forth in claim 70 whereinthe amine antioxidant is a diaryl amine.
 100. A fluid composition as setforth in claim 19 wherein said epoxide acid scavenger is selected fromthe group consisting of a derivative of a 3,4-epoxy cyclohexanecarboxylate and a diepoxide compound corresponding to the formula

wherein R ³ is an organic group containing 1 to 10 carbon atoms, from 0to 6 oxygen atoms and from 0 to 6 nitrogen atoms, and R ⁴ through R ⁹are independently selected from among hydrogen and aliphatic groupscontaining 1 to 5 carbon atoms, and mixtures of the 3,4-epoxycyclohexane carboxylate and the diepoxide compound.
 101. A fluidcomposition as set forth in claim 31 wherein said epoxide acid scavengeris selected from the group consisting of a derivative of a 3,4-epoxycyclohexane carboxylate and a diepoxide compound corresponding to theformula

wherein R ³ is an organic group containing 1 to 10 carbon atoms, from 0to 6 oxygen atoms and from 0 to 6 nitrogen atoms, and R ⁴ through R ⁹are independently selected from among hydrogen and aliphatic groupscontaining 1 to 5 carbon atoms, and mixtures of the 3,4-epoxycyclohexane carboxylate and the diepoxide compound.
 102. A fluidcomposition as set forth in claim 56 wherein said epoxide acid scavengeris selected from the group consisting of a derivative of a 3,4-epoxycyclohexane carboxylate and a diepoxide compound corresponding to theformula

wherein R ³ is an organic group containing 1 to 10 carbon atoms, from 0to 6 oxygen atoms and from 0 to 6 nitrogen atoms, and R ⁴ through R ⁹are independently selected from among hydrogen and aliphatic groupscontaining 1 to 5 carbon atoms, and mixtures of the 3,4-epoxycyclohexane carboxylate and the diepoxide compound.
 103. A fluidcomposition as set forth in claim 67 wherein said epoxide acid scavengeris selected from the group consisting of a derivative of a 3,4-epoxycyclohexane carboxylate and a diepoxide compound corresponding to theformula

wherein R ³ is an organic group containing 1 to 10 carbon atoms, from 0to 6 oxygen atoms and from 0 to 6 nitrogen atoms, and R ⁴ through R ⁹are independently selected from among hydrogen and aliphatic groupscontaining 1 to 5 carbon atoms, and mixtures of the 3,4-epoxycyclohexane carboxylate and the diepoxide compound.
 104. A fluidcomposition as set forth in claim 7 wherein the base stock comprisesbetween about 20% and about 90 % by weight of a trialkyl phosphate,between about 0 % and about 70 % by weight of a dialkyl aryl phosphateand between about 0 % and about 25 % by weight of an alkyl diarylphosphate.
 105. A fluid composition as set forth in claim 104 whereinthe base stock comprises between about 50% and about 90 % by weight of atrialkyl phosphate, between about 0 % and about 70 % by weight of adialkyl aryl phosphate and between about 0 % and about 25 % by weight ofan alkyl diaryl phosphate.
 106. A fluid composition as set forth inclaim 8 wherein the base stock comprises between about 20% and about 90% by weight of a trialkyl phosphate, between about 0 % and about 70 % byweight of a dialkyl aryl phosphate and between about 0 % and about 25 %by weight of an alkyl diaryl phosphate.
 107. A fluid composition as setforth in claim 106 wherein the base stock comprises between about 50%and about 90 % by weight of a trialkyl phosphate, between about 0 % andabout 70 % by weight of a dialkyl aryl phosphate and between about 0 %and about 25 % by weight of an alkyl diaryl phosphate.
 108. A fluidcomposition as set forth in claim 19 wherein the base stock comprisesbetween about 20% and about 90 % by weight of a trialkyl phosphate,between about 0 % and about 70 % by weight of a dialkyl aryl phosphateand between about 0 % and about 25 % by weight of an alkyl diarylphosphate.
 109. A fluid composition as set forth in claim 108 whereinthe base stock comprises between about 50% and about 90 % by weight of atrialkyl phosphate, between about 0 % and about 70 % by weight of adialkyl aryl phosphate and between about 0 % and about 25 % by weight ofan alkyl diaryl phosphate.
 110. A fluid composition as set forth inclaim 31 wherein the base stock comprises between about 20% and about 90% by weight of a trialkyl phosphate, between about 0 % and about 70 % byweight of a dialkyl aryl phosphate and between about 0 % and about 25 %by weight of an alkyl diaryl phosphate.
 111. A fluid composition as setforth in claim 110 wherein the base stock comprises between about 50%and about 90 % by weight of a trialkyl phosphate, between about 0 % andabout 70 % by weight of a dialkyl aryl phosphate and between about 0 %and about 25 % by weight of an alkyl diaryl phosphate.
 112. A fluidcomposition as set forth in claim 50 wherein the base stock comprisesbetween about 20% and about 99 % by weight of a trialkyl phosphate,between about 0 % and about 70 % by weight of a dialkyl aryl phosphateand between about 0 % and about 25 % by weight of an alkyl diarylphosphate.
 113. A fluid composition as set forth in claim 112 whereinthe base stock comprises between about 50% and about 99 % by weight of atrialkyl phosphate, between about 0 % and about 70 % by weight of adialkyl aryl phosphate and between about 0 % and about 25 % by weight ofan alkyl diaryl phosphate.
 114. A fluid composition as set forth inclaim 56 wherein the base stock comprises between about 20% and about 99% by weight of a trialkyl phosphate, between about 0 % and about 70 % byweight of a dialkyl aryl phosphate and between about 0 % and about 25 %by weight of an alkyl diaryl phosphate.
 115. A fluid composition as setforth in claim 114 wherein the base stock comprises between about 50%and about 99 % by weight of a trialkyl phosphate, between about 0 % andabout 70 % by weight of a dialkyl aryl phosphate and between about 0 %and about 25 % by weight of an alkyl diaryl phosphate.
 116. A fluidcomposition as set forth in claim 67 wherein the base stock comprisesbetween about 35% and about 99 % by weight of a trialkyl phosphate,between about 0 % and about 35 % by weight of a dialkyl aryl phosphateand between about 0 % and about 20 % by weight of an triaryl phosphate.117. A fluid composition as set forth in claim 116 wherein the basestock comprises between about 80% and about 99 % by weight of a trialkylphosphate, and between about 1 % and about 20 % by weight of an triarylphosphate.
 118. A fluid composition as set forth in any of claims 1, 19,31, 50, 56 or 67 wherein in the anti-erosion additive comprising analkali metal salt of a perfluoroalkyl sulfonic acid the alkylsubstituent comprises from 5 to 12 carbon atoms.